<PAGE>   1
As filed with the Securities and Exchange Commission on September 7, 2000


                                                     Registration No. 333-______
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                                   ----------

                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                                  -------------

                           ARENA PHARMACEUTICALS, INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                                     23-2908305
(State or other jurisdiction of            (I.R.S. Employer Identification No.)
 incorporation or organization)

        6166 NANCY RIDGE DRIVE
             SAN DIEGO, CA                                      92121
(Address of principal executive offices)                     (Zip Code)


            ARENA PHARMACEUTICALS, INC. 2000 EQUITY COMPENSATION PLAN
                            (Full title of the plan)


                                    JACK LIEF
                           ARENA PHARMACEUTICALS, INC.
                             6166 NANCY RIDGE DRIVE
                               SAN DIEGO, CA 92121
                     (Name and address of agent for service)

                                 (858) 453-7200
          (Telephone number, including area code, of agent for service)

                                  ------------

                        Copies of all communications to:
                             PETER P. WALLACE, ESQ.
                             STEVEN W. SPECTOR, ESQ.
                           MORGAN, LEWIS & BOCKIUS LLP
                         300 S. GRAND AVENUE, 22ND FLOOR
                              LOS ANGELES, CA 90071
                                 (213) 612-2500

                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
                                                                      Proposed Maximum       Proposed Maximum          Amount of
                                                  Amount to be       Offering Price Per     Aggregate Offering        Registration
     Title of Securities to be Registered        Registered(1)            Share(2)              Price(2)                 Fee(3)
<S>                                             <C>                  <C>                    <C>                       <C>
   Common Stock, par value $0.0001 per share    2,000,000 shares           $34.656             $69,312,000             $18,298.37
</TABLE>

(1)     This Registration Statement shall cover shares of Common Stock of Arena
        Pharmaceuticals which are offered or sold pursuant to the plan named
        above. Pursuant to Rule 416(a) of the Securities Act of 1933, this
        Registration Statement shall also cover any additional shares of Common
        Stock which are offered or issued under the plan to prevent dilution
        resulting from stock splits, stock dividends or similar transactions.

(2)     Estimated pursuant to paragraphs (c) and (h) of Rule 457 solely for the
        purpose of calculating the registration fee, based upon the average of
        the reported high and low sales prices for a share of Common Stock of
        Arena Pharmaceuticals, Inc. on August 30, 2000 as reported in the Nasdaq
        National Market.

(3)     Calculated pursuant to Section 6(b) of the Securities Act of 1933 as
        follows: proposed maximum aggregated offering price multiplied by
        $264.00/$1,000,000.
================================================================================


<PAGE>   2

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

The following documents filed by Arena Pharmaceuticals, Inc. (the "Company")
with the Securities and Exchange Commission (the "Commission"), are incorporated
by reference into this Registration Statement.

        1.      The Company's Prospectus covering the offer and sale of shares
                of the Company's common stock, par value $0.0001 per share (the
                "Common Stock"), filed with the Commission on July 28, 2000
                pursuant to Rule 424(b) under the Securities Act of 1933 (the
                "Securities Act"), which contains audited financial statements
                for the Company's latest fiscal year for which such statements
                have been filed (File No. 333-35944).

        2.      The description of the shares of the Company's Common Stock
                contained in the Company's Registration Statement on Form 8-A
                filed under the Securities Exchange Act of 1934 (the "Exchange
                Act") with the Commission on July 26, 2000, including all
                amendments and reports updating such description (File No.
                333-35944).

All documents subsequently filed by the Company with the Commission pursuant to
Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act after the date of this
Registration Statement, but prior to the filing of a post-effective amendment to
this Registration Statement which indicate that all securities offered by this
Registration Statement have been sold or which deregisters all such securities
then remaining unsold, shall be deemed to be incorporated by reference into this
Registration Statement. Each document incorporated by reference into this
Registration Statement shall be deemed to be a part of this Registration
Statement from the date of the filing of such document with the Commission until
the information contained therein is superseded or updated by any subsequently
filed document which is incorporated by reference into this Registration
Statement or by any document which constitutes part of the prospectus relating
to the Arena Pharmaceuticals, Inc. 2000 Equity Compensation Plan and meets the
requirements of Section 10(a) of the Securities Act.

Any statement contained in a document incorporated by reference herein shall be
deemed to be modified or superseded for purposes hereof to the extent that a
statement contained herein (or in any other subsequently filed document that is
also incorporated by reference herein) modifies or supersedes such statement.
Any such statement so modified or superseded shall not be deemed, except as so
modified or superseded, to constitute a part hereof.

ITEM 4. DESCRIPTION OF SECURITIES.

The class of securities offered under this Registration Statement is registered
under Section 12(g) of the Exchange Act.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

Not Applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

        Article IV of the Company's by-laws provide that it shall, to the full
extent authorized or permitted by applicable law, indemnify any current or
former director or officer. Subject to applicable law, the Company may indemnify
an employee or agent to the extent that the board of directors or stockholders
may determine in its or their discretion.

        Section 145 of the General Corporation Law of the State of Delaware (the
"DGCL") permits a corporation, under specified circumstances, to indemnify its
directors, officers, employees or agents against expenses (including attorney's
fees), judgments, fines and amounts paid in settlements actually and reasonably
incurred by them in connection with any action, suit or proceeding brought by
third parties by reason of the fact that they were or are directors, officers,
employees or agents of the corporation or


<PAGE>   3

were or are serving at the request of the corporation as a director, officer,
employee or agent of another corporation, if such directors, officers,
employees, or agents acted in good faith and in a manner they reasonably
believed to be in or not opposed to the best interests of the corporation and,
with respect to any criminal action or proceeding, had no reasonable cause to
believe their conduct was unlawful. In a derivative action, i.e., one by or in
the right of the corporation, indemnification may be made only for expenses
actually and reasonably incurred by directors, officers, employees or agents in
connection with the defense or settlement of an action or suit, and only with
respect to a matter as to which they shall have acted in good faith and in a
manner they reasonably believed to be in or not opposed to the best interests of
the corporation, except that no indemnification shall be made in respect of any
claim issue or matter as to which such person shall be adjudicated to be liable
to the corporation unless and the court in which the action or suit was brought
shall determine upon application that the defendant directors, officers,
employees, or agents are fairly and reasonably entitled to indemnification for
such expenses in view of all of the circumstances of the case, despite such
adjudication of liability. Expenses incurred (including attorney's fees) by an
officer or director in defending any civil, criminal, administrative or
investigative action, suit or proceeding may be paid by the corporation in
advance of the final disposition of such action, suit or proceeding upon receipt
of an undertaking by or on behalf of such director or officer to repay such
amount if it shall ultimately be determined that he is not entitled to be
indemnified by the corporation.

        Article IV of the Company's Certificate of Incorporation, as amended,
provides that a director shall not be personally liable to the Company or its
stockholders for monetary damages for breach of fiduciary duty as a director,
except for liability (a) for any breach of the director's duty of loyalty to the
Company or its stockholders, (b) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (c) pursuant
to Section 174 of the DGCL, or (d) for any transaction from which a director
derived an improper personal benefit.

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

Not Applicable.

ITEM 8. EXHIBITS.

The following exhibits are filed herewith or incorporated by reference as part
of this Registration Statement:


<TABLE>
<CAPTION>
             EXHIBIT
               NO.                            DESCRIPTION
               ---                            -----------
<S>                       <C>
               4.1        Arena Pharmaceuticals, Inc. Certificate of Incorporation, as amended(1).

               4.2        Arena Pharmaceuticals, Inc. Amended and Restated Bylaws(1).

               4.3        Arena Pharmaceuticals, Inc. 2000 Equity Compensation Plan(1).

               5.1        Opinion of Morgan, Lewis & Bockius, LLP, counsel to the Company(2).

              23.1        Consent of Ernst & Young LLP(2).

              24.1        Power of Attorney (set forth on signature page to this Registration Statement).
</TABLE>

--------------------------------------------------------------------------------

        (1)  Incorporated herein by reference to Registration Statement No.
             333-35944

        (2)  Filed herewith


<PAGE>   4

ITEM 9. UNDERTAKINGS.

The undersigned registrant hereby undertakes:

(a)     (1) To file, during any period in which offers or sales are being made,
a post-effective amendment to this registration statement:

                (i)     To include any prospectus required by Section 10(a)(3)
                        of the Securities Act;

                (ii)    To reflect in the prospectus any facts or events arising
                        after the effective date of the registration statement
                        (or the most recent post-effective amendment thereof)
                        which, individually or in the aggregate, represent a
                        fundamental change in the information set forth in the
                        registration statement. Notwithstanding the foregoing,
                        any increase or decrease in volume of securities offered
                        (if the total dollar value of securities offered would
                        not exceed that which was registered) and any deviation
                        from the low or high end of the estimated maximum
                        offering range may be reflected in the form of
                        prospectus filed with the Commission pursuant to Rule
                        424(b) if, in the aggregate, the changes in volume and
                        price represent no more than 20 percent change in the
                        maximum aggregate offering price set forth in the
                        "Calculation of Registration Fee" table in the effective
                        registration statement;

                (iii)   To include any material information with respect to the
                        plan of distribution not previously disclosed in the
                        registration statement or any material change to such
                        information in the registration statement.

        (2) That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

        (3) To remove from registration by means of a post-effective amendment
any of the securities being registered which remain unsold at the termination of
the offering.

(b) The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act that is incorporated by reference in the registration statement
shall be deemed to be a new registration statement relating to the securities
offered therein, and the offering of such securities at that time shall be
deemed to be the initial bona fide offering thereof.

(h) Insofar as indemnification for liabilities arising under the Securities Act
may be permitted to directors, officers and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Commission such indemnification is
against public policy as expressed in the Act and is, therefore, unenforceable.
In the event that a claim for indemnification against such liabilities (other
than the payment by the registrant of expenses incurred or paid by a director,
officer or controlling person of the registrant in the successful defense of any
action, suit or proceeding) is asserted by such director, officer or controlling
person in connection with the securities being registered, the registrant will,
unless in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the Act and
will be governed by the final adjudication of such issue.

(i)(1) For purposes of determining any liability under the Securities Act, the
information omitted from the form of prospectus filed as part of this
registration statement in reliance upon Rule 430A and contained in a form of
prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h)
under the Securities Act shall be deemed to be part of this registration
statement as of the time it was declared effective.

(2) For the purpose of determining any liability under the Securities Act, each
post-effective amendment that contains a form of prospectus shall be deemed to
be a new registration statement relating to the securities offered therein, and
the offering of such securities at that time shall be deemed to be the initial
bona fide offering thereof.


<PAGE>   5

                                   SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the
registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of San Diego, State of California, on August 29, 2000.

                                       ARENA PHARMACEUTICALS, INC.

                                       By: /s/ Jack Lief
                                           ------------------------------------
                                           Jack Lief
                                           President and Chief Executive Officer


        KNOW ALL PERSONS BY THESE PRESENTS, that each of the undersigned
directors and officers of Arena Pharmaceuticals, Inc. hereby constitutes and
appoints Jack Lief, and each of them, his true and lawful attorneys-in-fact and
agents, for him and in his name, place and stead, in any and all capacities, to
sign one or more amendments to this Registration Statement on Form S-8 under the
Securities Act, including post-effective amendments and other related documents,
and to file the same with the Commission under the Securities Act, hereby
granting power and authority to do and perform any and all acts and things
requisite and necessary to be done in and about the premises, as fully as to all
intents and purposes as he might or could do in person, hereby ratifying and
confirming all that said attorneys-in-fact and agents may lawfully do or cause
to be done by virtue hereof

        Pursuant to the requirements of the Securities Act, this Registration
Statement and the foregoing Power of Attorney have been signed by the following
persons in the capacities indicated on August 29, 2000:

<TABLE>
<CAPTION>
            Signature                                       Title
            ---------                                       -----
<S>                                            <C>
/s/ Jack Lief                                  President, Chief Executive Officer and
------------------------------------           Director
Jack Lief

/s/ Dominic P. Behan                           Vice President, Research and Director
------------------------------------
Dominic P. Behan

/s/ Derek T. Chalmers                          Vice President, Research and Director
------------------------------------
Derek T. Chalmers

/s/ Robert Hoffman                             Vice President, Finance
------------------------------------           (principal accounting officer)
Robert Hoffman

/s/ John P. McAlister, III                     Director
------------------------------------
John P. McAlister, III

/s/ Michael Steinmetz                          Director
------------------------------------
Michael Steinmetz

/s/ Stefan Ryser                               Director
------------------------------------
Stefan Ryser
</TABLE>


<PAGE>   6

                                INDEX TO EXHIBITS

<TABLE>
<CAPTION>
             EXHIBIT
               NO.                            DESCRIPTION
               ---                            -----------
<S>                       <C>
               4.1        Arena Pharmaceuticals, Inc. Certificate of Incorporation, as amended(1).

               4.2        Arena Pharmaceuticals, Inc. Amended and Restated Bylaws(1).

               4.3        Arena Pharmaceuticals, Inc. 2000 Equity Compensation Plan(1).

               5.1        Opinion of Morgan, Lewis & Bockius, LLP, counsel to the Company(2).

              23.1        Consent of Ernst & Young LLP(2).

              24.1        Power of Attorney (set forth on signature page to this Registration Statement).
</TABLE>

--------------------------------------------------------------------------------

        (1)  Incorporated herein by reference to Registration Statement No.
             333-35944

        (2)  Filed herewith
[{"type": "S-8", "sequence": "1", "filename": "a65447s-8.txt", "description": "FORM S-8", "secsgml_size_bytes": 20264}]
{"company-data": {"conformed-name": "ARENA PHARMACEUTICALS INC", "cik": "0001080709", "assigned-sic": "2834", "irs-number": "232908305", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-45332", "film-number": "718364"}, "business-address": {"street1": "6166 NANCY RIDGE DR", "city": "SAN DIEGO", "state": "CA", "zip": "92121", "phone": "8584537200"}, "mail-address": {"street1": "6166 NANCY RIDGE DR", "city": "SAN DIEGO", "state": "CA", "zip": "92121"}}
1
As filed with the Securities and Exchange Commission on September 7, 2000

                                                     Registration No. 333-______
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                                   ----------

                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                                  -------------

                           ARENA PHARMACEUTICALS, INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                                     23-2908305
(State or other jurisdiction of            (I.R.S. Employer Identification No.)
 incorporation or organization)

        6166 NANCY RIDGE DRIVE
             SAN DIEGO, CA                                      92121
(Address of principal executive offices)                     (Zip Code)

            ARENA PHARMACEUTICALS, INC. 2000 EQUITY COMPENSATION PLAN
                            (Full title of the plan)

                                    JACK LIEF
                           ARENA PHARMACEUTICALS, INC.
                             6166 NANCY RIDGE DRIVE
                               SAN DIEGO, CA 92121
                     (Name and address of agent for service)

                                 (858) 453-7200
          (Telephone number, including area code, of agent for service)

                                  ------------

                        Copies of all communications to:
                             PETER P. WALLACE, ESQ.
                             STEVEN W. SPECTOR, ESQ.
                           MORGAN, LEWIS & BOCKIUS LLP
                         300 S. GRAND AVENUE, 22ND FLOOR
                              LOS ANGELES, CA 90071
                                 (213) 612-2500

                         CALCULATION OF REGISTRATION FEE
Proposed Maximum       Proposed Maximum          Amount of
                                                  Amount to be       Offering Price Per     Aggregate Offering        Registration
     Title of Securities to be Registered        Registered(1)            Share(2)              Price(2)                 Fee(3)
Common Stock, par value $0.0001 per share    2,000,000 shares           $34.656             $69,312,000             $18,298.37
(1)     This Registration Statement shall cover shares of Common Stock of Arena
        Pharmaceuticals which are offered or sold pursuant to the plan named
        above. Pursuant to Rule 416(a) of the Securities Act of 1933, this
        Registration Statement shall also cover any additional shares of Common
        Stock which are offered or issued under the plan to prevent dilution
        resulting from stock splits, stock dividends or similar transactions.

(2)     Estimated pursuant to paragraphs (c) and (h) of Rule 457 solely for the
        purpose of calculating the registration fee, based upon the average of
        the reported high and low sales prices for a share of Common Stock of
        Arena Pharmaceuticals, Inc. on August 30, 2000 as reported in the Nasdaq
        National Market.

(3)     Calculated pursuant to Section 6(b) of the Securities Act of 1933 as
        follows: proposed maximum aggregated offering price multiplied by
        $264.00/$1,000,000.
================================================================================
2

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

The following documents filed by Arena Pharmaceuticals, Inc. (the "Company")
with the Securities and Exchange Commission (the "Commission"), are incorporated
by reference into this Registration Statement.

        1.      The Company's Prospectus covering the offer and sale of shares
                of the Company's common stock, par value $0.0001 per share (the
                "Common Stock"), filed with the Commission on July 28, 2000
                pursuant to Rule 424(b) under the Securities Act of 1933 (the
                "Securities Act"), which contains audited financial statements
                for the Company's latest fiscal year for which such statements
                have been filed (File No. 333-35944).

        2.      The description of the shares of the Company's Common Stock
                contained in the Company's Registration Statement on Form 8-A
                filed under the Securities Exchange Act of 1934 (the "Exchange
                Act") with the Commission on July 26, 2000, including all
                amendments and reports updating such description (File No.
                333-35944).

All documents subsequently filed by the Company with the Commission pursuant to
Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act after the date of this
Registration Statement, but prior to the filing of a post-effective amendment to
this Registration Statement which indicate that all securities offered by this
Registration Statement have been sold or which deregisters all such securities
then remaining unsold, shall be deemed to be incorporated by reference into this
Registration Statement. Each document incorporated by reference into this
Registration Statement shall be deemed to be a part of this Registration
Statement from the date of the filing of such document with the Commission until
the information contained therein is superseded or updated by any subsequently
filed document which is incorporated by reference into this Registration
Statement or by any document which constitutes part of the prospectus relating
to the Arena Pharmaceuticals, Inc. 2000 Equity Compensation Plan and meets the
requirements of Section 10(a) of the Securities Act.

Any statement contained in a document incorporated by reference herein shall be
deemed to be modified or superseded for purposes hereof to the extent that a
statement contained herein (or in any other subsequently filed document that is
also incorporated by reference herein) modifies or supersedes such statement.
Any such statement so modified or superseded shall not be deemed, except as so
modified or superseded, to constitute a part hereof.

ITEM 4. DESCRIPTION OF SECURITIES.

The class of securities offered under this Registration Statement is registered
under Section 12(g) of the Exchange Act.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

Not Applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

        Article IV of the Company's by-laws provide that it shall, to the full
extent authorized or permitted by applicable law, indemnify any current or
former director or officer. Subject to applicable law, the Company may indemnify
an employee or agent to the extent that the board of directors or stockholders
may determine in its or their discretion.

        Section 145 of the General Corporation Law of the State of Delaware (the
"DGCL") permits a corporation, under specified circumstances, to indemnify its
directors, officers, employees or agents against expenses (including attorney's
fees), judgments, fines and amounts paid in settlements actually and reasonably
incurred by them in connection with any action, suit or proceeding brought by
third parties by reason of the fact that they were or are directors, officers,
employees or agents of the corporation or
3

were or are serving at the request of the corporation as a director, officer,
employee or agent of another corporation, if such directors, officers,
employees, or agents acted in good faith and in a manner they reasonably
believed to be in or not opposed to the best interests of the corporation and,
with respect to any criminal action or proceeding, had no reasonable cause to
believe their conduct was unlawful. In a derivative action, i.e., one by or in
the right of the corporation, indemnification may be made only for expenses
actually and reasonably incurred by directors, officers, employees or agents in
connection with the defense or settlement of an action or suit, and only with
respect to a matter as to which they shall have acted in good faith and in a
manner they reasonably believed to be in or not opposed to the best interests of
the corporation, except that no indemnification shall be made in respect of any
claim issue or matter as to which such person shall be adjudicated to be liable
to the corporation unless and the court in which the action or suit was brought
shall determine upon application that the defendant directors, officers,
employees, or agents are fairly and reasonably entitled to indemnification for
such expenses in view of all of the circumstances of the case, despite such
adjudication of liability. Expenses incurred (including attorney's fees) by an
officer or director in defending any civil, criminal, administrative or
investigative action, suit or proceeding may be paid by the corporation in
advance of the final disposition of such action, suit or proceeding upon receipt
of an undertaking by or on behalf of such director or officer to repay such
amount if it shall ultimately be determined that he is not entitled to be
indemnified by the corporation.

        Article IV of the Company's Certificate of Incorporation, as amended,
provides that a director shall not be personally liable to the Company or its
stockholders for monetary damages for breach of fiduciary duty as a director,
except for liability (a) for any breach of the director's duty of loyalty to the
Company or its stockholders, (b) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (c) pursuant
to Section 174 of the DGCL, or (d) for any transaction from which a director
derived an improper personal benefit.

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

Not Applicable.

ITEM 8. EXHIBITS.

The following exhibits are filed herewith or incorporated by reference as part
of this Registration Statement:
EXHIBIT
               NO.                            DESCRIPTION
               ---                            -----------
4.1        Arena Pharmaceuticals, Inc. Certificate of Incorporation, as amended(1).

               4.2        Arena Pharmaceuticals, Inc. Amended and Restated Bylaws(1).

               4.3        Arena Pharmaceuticals, Inc. 2000 Equity Compensation Plan(1).

               5.1        Opinion of Morgan, Lewis & Bockius, LLP, counsel to the Company(2).

              23.1        Consent of Ernst & Young LLP(2).

              24.1        Power of Attorney (set forth on signature page to this Registration Statement).
--------------------------------------------------------------------------------

        (1)  Incorporated herein by reference to Registration Statement No.
             333-35944

        (2)  Filed herewith
4

ITEM 9. UNDERTAKINGS.

The undersigned registrant hereby undertakes:

(a)     (1) To file, during any period in which offers or sales are being made,
a post-effective amendment to this registration statement:

                (i)     To include any prospectus required by Section 10(a)(3)
                        of the Securities Act;

                (ii)    To reflect in the prospectus any facts or events arising
                        after the effective date of the registration statement
                        (or the most recent post-effective amendment thereof)
                        which, individually or in the aggregate, represent a
                        fundamental change in the information set forth in the
                        registration statement. Notwithstanding the foregoing,
                        any increase or decrease in volume of securities offered
                        (if the total dollar value of securities offered would
                        not exceed that which was registered) and any deviation
                        from the low or high end of the estimated maximum
                        offering range may be reflected in the form of
                        prospectus filed with the Commission pursuant to Rule
                        424(b) if, in the aggregate, the changes in volume and
                        price represent no more than 20 percent change in the
                        maximum aggregate offering price set forth in the
                        "Calculation of Registration Fee" table in the effective
                        registration statement;

                (iii)   To include any material information with respect to the
                        plan of distribution not previously disclosed in the
                        registration statement or any material change to such
                        information in the registration statement.

        (2) That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

        (3) To remove from registration by means of a post-effective amendment
any of the securities being registered which remain unsold at the termination of
the offering.

(b) The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act that is incorporated by reference in the registration statement
shall be deemed to be a new registration statement relating to the securities
offered therein, and the offering of such securities at that time shall be
deemed to be the initial bona fide offering thereof.

(h) Insofar as indemnification for liabilities arising under the Securities Act
may be permitted to directors, officers and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Commission such indemnification is
against public policy as expressed in the Act and is, therefore, unenforceable.
In the event that a claim for indemnification against such liabilities (other
than the payment by the registrant of expenses incurred or paid by a director,
officer or controlling person of the registrant in the successful defense of any
action, suit or proceeding) is asserted by such director, officer or controlling
person in connection with the securities being registered, the registrant will,
unless in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the Act and
will be governed by the final adjudication of such issue.

(i)(1) For purposes of determining any liability under the Securities Act, the
information omitted from the form of prospectus filed as part of this
registration statement in reliance upon Rule 430A and contained in a form of
prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h)
under the Securities Act shall be deemed to be part of this registration
statement as of the time it was declared effective.

(2) For the purpose of determining any liability under the Securities Act, each
post-effective amendment that contains a form of prospectus shall be deemed to
be a new registration statement relating to the securities offered therein, and
the offering of such securities at that time shall be deemed to be the initial
bona fide offering thereof.
5

                                   SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the
registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of San Diego, State of California, on August 29, 2000.

                                       ARENA PHARMACEUTICALS, INC.

                                       By: /s/ Jack Lief
                                           ------------------------------------
                                           Jack Lief
                                           President and Chief Executive Officer

        KNOW ALL PERSONS BY THESE PRESENTS, that each of the undersigned
directors and officers of Arena Pharmaceuticals, Inc. hereby constitutes and
appoints Jack Lief, and each of them, his true and lawful attorneys-in-fact and
agents, for him and in his name, place and stead, in any and all capacities, to
sign one or more amendments to this Registration Statement on Form S-8 under the
Securities Act, including post-effective amendments and other related documents,
and to file the same with the Commission under the Securities Act, hereby
granting power and authority to do and perform any and all acts and things
requisite and necessary to be done in and about the premises, as fully as to all
intents and purposes as he might or could do in person, hereby ratifying and
confirming all that said attorneys-in-fact and agents may lawfully do or cause
to be done by virtue hereof

        Pursuant to the requirements of the Securities Act, this Registration
Statement and the foregoing Power of Attorney have been signed by the following
persons in the capacities indicated on August 29, 2000:
Signature                                       Title
            ---------                                       -----
/s/ Jack Lief                                  President, Chief Executive Officer and
------------------------------------           Director
Jack Lief

/s/ Dominic P. Behan                           Vice President, Research and Director
------------------------------------
Dominic P. Behan

/s/ Derek T. Chalmers                          Vice President, Research and Director
------------------------------------
Derek T. Chalmers

/s/ Robert Hoffman                             Vice President, Finance
------------------------------------           (principal accounting officer)
Robert Hoffman

/s/ John P. McAlister, III                     Director
------------------------------------
John P. McAlister, III

/s/ Michael Steinmetz                          Director
------------------------------------
Michael Steinmetz

/s/ Stefan Ryser                               Director
------------------------------------
Stefan Ryser
6

                                INDEX TO EXHIBITS
EXHIBIT
               NO.                            DESCRIPTION
               ---                            -----------
4.1        Arena Pharmaceuticals, Inc. Certificate of Incorporation, as amended(1).

               4.2        Arena Pharmaceuticals, Inc. Amended and Restated Bylaws(1).

               4.3        Arena Pharmaceuticals, Inc. 2000 Equity Compensation Plan(1).

               5.1        Opinion of Morgan, Lewis & Bockius, LLP, counsel to the Company(2).

              23.1        Consent of Ernst & Young LLP(2).

              24.1        Power of Attorney (set forth on signature page to this Registration Statement).
--------------------------------------------------------------------------------

        (1)  Incorporated herein by reference to Registration Statement No.
             333-35944

        (2)  Filed herewith