<PAGE>   1
              As filed with the Securities and Exchange Commission
                                on March 26, 1998
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8

                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933

                                  SEPRACOR INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                                          22-2536587
(State or other jurisdiction of                            (I.R.S. Employer
 incorporation or organization)                           Identification No.)

                111 LOCKE DRIVE, MARLBOROUGH, MASSACHUSETTS 01752
               (Address of Principal Executive Offices) (Zip Code)

                             1997 STOCK OPTION PLAN
                            (Full title of the plan)

                              MARK G. BORDEN, ESQ.
                                HALE AND DORR LLP
                                 60 STATE STREET
                           BOSTON, MASSACHUSETTS 02109
                     (Name and address of agent for service)

                                 (617) 526-6000
          (Telephone number, including area code, of agent for service)

                         CALCULATION OF REGISTRATION FEE

================================================================================
<TABLE>
<CAPTION>
                                              Proposed          Proposed
    Title of                                   maximum          maximum
securities to be        Amount to be       offering price      aggregate           Amount of
   registered            registered           per share      offering price    registration fee
----------------       --------------      --------------    --------------    ----------------
<S>                    <C>                  <C>              <C>                   <C>
Common Stock,
$.10 par value         500,000 shares       $42.56(1)        $21,280,000(1)        $6,278
-----------------------------------------------------------------------------------------------
</TABLE>
(1)      Estimated solely for the purpose of calculating the registration fee,
         and based upon the average of the high and low prices of the Common
         Stock on the Nasdaq National Market on March 24, 1998 in accordance
         with Rules 457(c) and 457(h) of the Securities Act of 1933.
================================================================================





<PAGE>   2



PART I.  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

         The information required by Part I is included in documents sent or
given to participants in the 1997 Stock Option Plan of Sepracor Inc., a Delaware
corporation (the "Registrant") pursuant to Rule 428(b)(1) under the Securities
Act of 1933, as amended (the "Securities Act").

PART II.  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

         Item 3.  INCORPORATION OF DOCUMENTS BY REFERENCE.

         The Registrant is subject to the informational and reporting
requirements of Sections 13(a), 14, and 15(d) of the Securities Exchange Act of
1934, as amended (the "Exchange Act"), and in accordance therewith files
reports, proxy statements and other information with the Securities and Exchange
Commission. The following documents, which are filed with the Securities and
Exchange Commission, are incorporated in this Prospectus by reference:

                  (1) The Registrant's latest annual report filed pursuant to
         Section 13(a) or 15(d) of the Exchange Act, or the latest prospectus
         filed pursuant to Rule 424(b) under the Securities Act, that contains
         audited financial statements for the Registrant's latest fiscal year
         for which such statements have been filed.

                  (2) All other reports filed pursuant to Section 13(a) or 15(d)
         of the Exchange Act since the end of the fiscal year covered by the
         document referred to in (1) above.

                  (3) The description of the Common Stock, par value $.10 per
         share ("Common Stock"), contained in a registration statement filed
         under the Exchange Act, including any amendment or report filed for the
         purpose of updating such description.

         All documents subsequently filed by the Registrant pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a
post-effective amendment which indicates that all shares of Common Stock offered
hereby have been sold or which deregisters all shares of Common Stock then
remaining unsold, shall be deemed to be incorporated by reference herein and to
be part hereof from the date of the filing of such documents.

         Item 4.  DESCRIPTION OF SECURITIES.

         Not applicable.

                                      - 2 -


<PAGE>   3



         Item 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL.

         Not applicable.

         Item 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         Section 145 of the Delaware General Corporation Law provides that a
corporation may indemnify any person who was or is a party or is threatened to
be made a party to any threatened, pending or completed action or proceeding,
whether civil, criminal, administrative or investigative, by reason of the fact
that he is or was a director, officer, employee or agent of the corporation or
is or was serving at its request in such capacity in another corporation or
business association, against expenses (including attorneys' fees), judgments,
fines and amounts paid in settlement actually and reasonably incurred by him in
connection with such action, suit or proceeding if he acted in good faith and in
a manner he 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 his conduct was unlawful.

         Section 102(b)(7) of the Delaware General Corporation Law permits a
corporation to provide in its certificate of incorporation that a director of
the corporation shall not be personally liable to the corporation or its
stockholders for monetary damages for breach of fiduciary duty as a director,
except for liability (i) for any breach of the director's duty of loyalty to the
corporation or its stockholders, (ii) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (iii) under
Section 174 of the Delaware General Corporation Law, or (iv) for any transaction
from which the director derived an improper personal benefit.

         Article NINTH of the Registrant's Restated Certificate of Incorporation
(as amended, the "Certificate of Incorporation") provides that no director of
the Registrant shall be liable for any breach of a fiduciary duty, except to the
extent that the Delaware General Corporation Law prohibits the elimination or
limitation of liability of directors for breach of fiduciary duty.

         Article THIRTEENTH of the Certificate of Incorporation provides that a
director or officer of the Registrant (a) shall be indemnified by the Registrant
against all costs, charges, expenses (including attorneys' fees), judgments,
fines and amounts paid in settlement incurred in connection with any litigation
or other legal proceeding (other than an action by or in the right of the
Registrant) brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, and, with respect to any criminal action or proceeding, has no
reasonable cause to believe his conduct was unlawful and (b) shall be
indemnified by


                                      - 3 -


<PAGE>   4


the Registrant against all costs, charges and expenses (including attorneys'
fees) incurred in connection with any action by or in the right of the
Registrant brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, except that no indemnification shall be made with respect to any
such matter as to which such person shall have been adjudged to be liable to the
Registrant, unless a court determines that, despite such adjudication but in
view of all of the circumstances, he is entitled to indemnification of such
expenses. Notwithstanding the foregoing, to the extent that a director or
officer has been successful, on the merits or otherwise, including, without
limitation, the dismissal of an action without prejudice or the settlement of an
action without admission of liability, he is required to be indemnified by the
Registrant against all costs, charges and expenses (including attorneys' fees)
incurred in connection therewith. Expenses shall be advanced to a director or
officer at his request, provided that he undertakes to repay the amount advanced
if it is ultimately determined that the is not entitled to indemnification for
such expenses.

         Indemnification is required to be made unless the Board of Directors or
independent legal counsel determines that the applicable standard of conduct
required for indemnification has not been met. In the event of a determination
by the Board of Directors or independent legal counsel that the director or
officer did not meet the applicable standard of conduct required for
indemnification, or if the Registrant fails to make the applicable standard of
conduct required for indemnification, or if the Registrant fails to make an
indemnification payment within 60 days after such payment is claimed by such
person, such person is permitted to petition the court to make an independent
determination as to whether such person is entitled to indemnification. As a
condition precedent to the right of indemnification, the director or officer
must give the Registrant notice of the action for which indemnity is sought and
the Registrant has the right to participate in such action or assume the defense
thereto.

         Article THIRTEENTH of the Certificate of Incorporation further provides
that the indemnification provided therein is not exclusive, and provides that in
the event that the Delaware General Corporation Law is amended to expand the
indemnification permitted to directors or officers the Registrant must indemnify
those persons to the fullest extent permitted by such law as so amended.

         Item 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

                                      - 4 -


<PAGE>   5



         Item 8.  EXHIBITS.

         The Exhibit Index immediately preceding the exhibits is incorporated
herein by reference.

         Item 9.  UNDERTAKINGS.

         1.       The Registrant hereby undertakes:

                  (a)      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; and

                  (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;

         PROVIDED, HOWEVER that paragraphs (i) and (ii) do not apply if the
         registration statement is on Form S-3 or Form S-8, and the information
         required to be included in a post-effective amendment by those
         paragraphs is contained in periodic reports filed by the Registrant
         pursuant to Section 13 or Section 15(d) of the Exchange Act that are
         incorporated by reference in the registration statement.

                  (b)      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.

                  (c)      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.

         2.       The Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's annual report

                                      - 5 -


<PAGE>   6



pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and, where
applicable, each filing of an employee benefit plan's annual report pursuant to
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 in the initial bona fide offering thereof.

         3.       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 Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities 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 Securities
Act and will be governed by the final adjudication of such issue.

                                      - 6 -


<PAGE>   7



                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, 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 Town of Marlborough, Commonwealth of Massachusetts on the
26th day of March, 1998.

                                             SEPRACOR INC.



                                             By: /s/ Timothy J. Barberich
                                                 -------------------------------
                                                 Timothy J. Barberich
                                                 President and
                                                 Chief Executive Officer


                                POWER OF ATTORNEY

         We, the undersigned officers and directors of Sepracor Inc. hereby
severally constitute Timothy J. Barberich, David P. Southwell and Mark G.
Borden, and each of them singly, our true and lawful attorneys with full power
to them, and each of them singly, to sign for us and in our names in the
capacities indicated below, the Registration Statement on Form S-8 filed
herewith and any and all subsequent amendments to said Registration Statement,
and generally to do all such things in our names and behalf in our capacities as
officers and directors to enable Sepracor Inc. to comply with all requirements
of the Securities and Exchange Commission, hereby ratifying and confirming our
signatures as they may be signed by said attorneys, or any of them, to said
Registration Statement and any and all amendments thereto.


                                      - 7 -


<PAGE>   8



         Pursuant to the requirements of the Securities Act, this Registration
Statement has been signed below by the following persons in the capacities and
on the date indicated.

<TABLE>
<CAPTION>
Signature                             Title                              Date
---------                             -----                              ----

<S>                                   <C>                                <C> 
/s/ Timothy J. Barberich              President; Chief Executive         March 26, 1998
----------------------------          Officer and Director 
Timothy J. Barberich                  (Principal Executive 
                                      Officer)             

/s/ David P. Southwell                Executive Vice President;          March 26, 1998
----------------------------          Chief Financial Officer      
David P. Southwell                    (Principal Financial Officer)

/s/ Robert F. Scumaci                 Senior Vice President,             March 26, 1998
----------------------------          Finance and Administration  
Robert F. Scumaci                     (Principal Accounting       
                                      Officer)                    

/s/James G. Andress                   Director                           March 26, 1998
----------------------------
James G. Andress

/s/ Digby W. Barrios                  Director                           March 26, 1998
----------------------------
Digby W. Barrios

/s/ Robert J. Cresci                  Director                           March 26, 1998
----------------------------
Robert J. Cresci

/s/ Robert F. Johnston                Director                           March 26, 1998
----------------------------
Robert F. Johnston
</TABLE>

                                      - 8 -


<PAGE>   9


<TABLE>
<S>                                   <C>                                <C> 
/s/ Keith Mansford                    Director                           March 26, 1998
----------------------------
Keith Mansford, Ph.D.

/s/ James F. Mrazek                   Director                           March 26, 1998
----------------------------
James F. Mrazek

/s/ Alan A. Steigrod                  Director                           March 26, 1998
----------------------------
Alan A. Steigrod
</TABLE>


                                      - 9 -


<PAGE>   10



                                  EXHIBIT INDEX

Exhibit
Number         Description
------         -----------

  4.1*         Certificate of Incorporation of the Registrant, as amended

  4.2*         By-Laws of the Registrant

  4.3*         Specimen Certificate of Common Stock of the Registrant

  5            Opinion of Hale and Dorr LLP

 23.1          Consent of Hale and Dorr LLP (included in Exhibit 5)

 23.2          Consent of Coopers & Lybrand L.L.P.

 24            Power of Attorney (included in the signature pages of this
               Registration Statement)











------------------
*  Incorporated herein by reference from the Registrant's Registration
   Statement on Form S-1 (File No. 33-41653).



                                     - 10 -
[{"type": "S-8", "sequence": "1", "description": "SEPRACOR INC", "secsgml_size_bytes": 20010}]
{"company-data": {"conformed-name": "SEPRACOR INC /DE/", "cik": "0000877357", "assigned-sic": "3826", "irs-number": "222536587", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-48719", "film-number": "98574804"}, "business-address": {"street1": "111 LOCKE DR", "city": "MARLBOROUGH", "state": "MA", "zip": "01757", "phone": "5084816700"}}
1
              As filed with the Securities and Exchange Commission
                                on March 26, 1998
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8

                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933

                                  SEPRACOR INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                                          22-2536587
(State or other jurisdiction of                            (I.R.S. Employer
 incorporation or organization)                           Identification No.)

                111 LOCKE DRIVE, MARLBOROUGH, MASSACHUSETTS 01752
               (Address of Principal Executive Offices) (Zip Code)

                             1997 STOCK OPTION PLAN
                            (Full title of the plan)

                              MARK G. BORDEN, ESQ.
                                HALE AND DORR LLP
                                 60 STATE STREET
                           BOSTON, MASSACHUSETTS 02109
                     (Name and address of agent for service)

                                 (617) 526-6000
          (Telephone number, including area code, of agent for service)

                         CALCULATION OF REGISTRATION FEE

================================================================================
Proposed          Proposed
    Title of                                   maximum          maximum
securities to be        Amount to be       offering price      aggregate           Amount of
   registered            registered           per share      offering price    registration fee
----------------       --------------      --------------    --------------    ----------------
Common Stock,
$.10 par value         500,000 shares       $42.56(1)        $21,280,000(1)        $6,278
-----------------------------------------------------------------------------------------------
(1)      Estimated solely for the purpose of calculating the registration fee,
         and based upon the average of the high and low prices of the Common
         Stock on the Nasdaq National Market on March 24, 1998 in accordance
         with Rules 457(c) and 457(h) of the Securities Act of 1933.
================================================================================
2

PART I.  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

         The information required by Part I is included in documents sent or
given to participants in the 1997 Stock Option Plan of Sepracor Inc., a Delaware
corporation (the "Registrant") pursuant to Rule 428(b)(1) under the Securities
Act of 1933, as amended (the "Securities Act").

PART II.  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

         Item 3.  INCORPORATION OF DOCUMENTS BY REFERENCE.

         The Registrant is subject to the informational and reporting
requirements of Sections 13(a), 14, and 15(d) of the Securities Exchange Act of
1934, as amended (the "Exchange Act"), and in accordance therewith files
reports, proxy statements and other information with the Securities and Exchange
Commission. The following documents, which are filed with the Securities and
Exchange Commission, are incorporated in this Prospectus by reference:

                  (1) The Registrant's latest annual report filed pursuant to
         Section 13(a) or 15(d) of the Exchange Act, or the latest prospectus
         filed pursuant to Rule 424(b) under the Securities Act, that contains
         audited financial statements for the Registrant's latest fiscal year
         for which such statements have been filed.

                  (2) All other reports filed pursuant to Section 13(a) or 15(d)
         of the Exchange Act since the end of the fiscal year covered by the
         document referred to in (1) above.

                  (3) The description of the Common Stock, par value $.10 per
         share ("Common Stock"), contained in a registration statement filed
         under the Exchange Act, including any amendment or report filed for the
         purpose of updating such description.

         All documents subsequently filed by the Registrant pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a
post-effective amendment which indicates that all shares of Common Stock offered
hereby have been sold or which deregisters all shares of Common Stock then
remaining unsold, shall be deemed to be incorporated by reference herein and to
be part hereof from the date of the filing of such documents.

         Item 4.  DESCRIPTION OF SECURITIES.

         Not applicable.

                                      - 2 -
3

         Item 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL.

         Not applicable.

         Item 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         Section 145 of the Delaware General Corporation Law provides that a
corporation may indemnify any person who was or is a party or is threatened to
be made a party to any threatened, pending or completed action or proceeding,
whether civil, criminal, administrative or investigative, by reason of the fact
that he is or was a director, officer, employee or agent of the corporation or
is or was serving at its request in such capacity in another corporation or
business association, against expenses (including attorneys' fees), judgments,
fines and amounts paid in settlement actually and reasonably incurred by him in
connection with such action, suit or proceeding if he acted in good faith and in
a manner he 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 his conduct was unlawful.

         Section 102(b)(7) of the Delaware General Corporation Law permits a
corporation to provide in its certificate of incorporation that a director of
the corporation shall not be personally liable to the corporation or its
stockholders for monetary damages for breach of fiduciary duty as a director,
except for liability (i) for any breach of the director's duty of loyalty to the
corporation or its stockholders, (ii) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (iii) under
Section 174 of the Delaware General Corporation Law, or (iv) for any transaction
from which the director derived an improper personal benefit.

         Article NINTH of the Registrant's Restated Certificate of Incorporation
(as amended, the "Certificate of Incorporation") provides that no director of
the Registrant shall be liable for any breach of a fiduciary duty, except to the
extent that the Delaware General Corporation Law prohibits the elimination or
limitation of liability of directors for breach of fiduciary duty.

         Article THIRTEENTH of the Certificate of Incorporation provides that a
director or officer of the Registrant (a) shall be indemnified by the Registrant
against all costs, charges, expenses (including attorneys' fees), judgments,
fines and amounts paid in settlement incurred in connection with any litigation
or other legal proceeding (other than an action by or in the right of the
Registrant) brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, and, with respect to any criminal action or proceeding, has no
reasonable cause to believe his conduct was unlawful and (b) shall be
indemnified by

                                      - 3 -
4

the Registrant against all costs, charges and expenses (including attorneys'
fees) incurred in connection with any action by or in the right of the
Registrant brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, except that no indemnification shall be made with respect to any
such matter as to which such person shall have been adjudged to be liable to the
Registrant, unless a court determines that, despite such adjudication but in
view of all of the circumstances, he is entitled to indemnification of such
expenses. Notwithstanding the foregoing, to the extent that a director or
officer has been successful, on the merits or otherwise, including, without
limitation, the dismissal of an action without prejudice or the settlement of an
action without admission of liability, he is required to be indemnified by the
Registrant against all costs, charges and expenses (including attorneys' fees)
incurred in connection therewith. Expenses shall be advanced to a director or
officer at his request, provided that he undertakes to repay the amount advanced
if it is ultimately determined that the is not entitled to indemnification for
such expenses.

         Indemnification is required to be made unless the Board of Directors or
independent legal counsel determines that the applicable standard of conduct
required for indemnification has not been met. In the event of a determination
by the Board of Directors or independent legal counsel that the director or
officer did not meet the applicable standard of conduct required for
indemnification, or if the Registrant fails to make the applicable standard of
conduct required for indemnification, or if the Registrant fails to make an
indemnification payment within 60 days after such payment is claimed by such
person, such person is permitted to petition the court to make an independent
determination as to whether such person is entitled to indemnification. As a
condition precedent to the right of indemnification, the director or officer
must give the Registrant notice of the action for which indemnity is sought and
the Registrant has the right to participate in such action or assume the defense
thereto.

         Article THIRTEENTH of the Certificate of Incorporation further provides
that the indemnification provided therein is not exclusive, and provides that in
the event that the Delaware General Corporation Law is amended to expand the
indemnification permitted to directors or officers the Registrant must indemnify
those persons to the fullest extent permitted by such law as so amended.

         Item 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

                                      - 4 -
5

         Item 8.  EXHIBITS.

         The Exhibit Index immediately preceding the exhibits is incorporated
herein by reference.

         Item 9.  UNDERTAKINGS.

         1.       The Registrant hereby undertakes:

                  (a)      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; and

                  (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;

         PROVIDED, HOWEVER that paragraphs (i) and (ii) do not apply if the
         registration statement is on Form S-3 or Form S-8, and the information
         required to be included in a post-effective amendment by those
         paragraphs is contained in periodic reports filed by the Registrant
         pursuant to Section 13 or Section 15(d) of the Exchange Act that are
         incorporated by reference in the registration statement.

                  (b)      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.

                  (c)      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.

         2.       The Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's annual report

                                      - 5 -
6

pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and, where
applicable, each filing of an employee benefit plan's annual report pursuant to
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 in the initial bona fide offering thereof.

         3.       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 Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities 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 Securities
Act and will be governed by the final adjudication of such issue.

                                      - 6 -
7

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, 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 Town of Marlborough, Commonwealth of Massachusetts on the
26th day of March, 1998.

                                             SEPRACOR INC.

                                             By: /s/ Timothy J. Barberich
                                                 -------------------------------
                                                 Timothy J. Barberich
                                                 President and
                                                 Chief Executive Officer

                                POWER OF ATTORNEY

         We, the undersigned officers and directors of Sepracor Inc. hereby
severally constitute Timothy J. Barberich, David P. Southwell and Mark G.
Borden, and each of them singly, our true and lawful attorneys with full power
to them, and each of them singly, to sign for us and in our names in the
capacities indicated below, the Registration Statement on Form S-8 filed
herewith and any and all subsequent amendments to said Registration Statement,
and generally to do all such things in our names and behalf in our capacities as
officers and directors to enable Sepracor Inc. to comply with all requirements
of the Securities and Exchange Commission, hereby ratifying and confirming our
signatures as they may be signed by said attorneys, or any of them, to said
Registration Statement and any and all amendments thereto.

                                      - 7 -
8

         Pursuant to the requirements of the Securities Act, this Registration
Statement has been signed below by the following persons in the capacities and
on the date indicated.
Signature                             Title                              Date
---------                             -----                              ----
/s/ Timothy J. Barberich              President; Chief Executive         March 26, 1998
----------------------------          Officer and Director 
Timothy J. Barberich                  (Principal Executive 
                                      Officer)             

/s/ David P. Southwell                Executive Vice President;          March 26, 1998
----------------------------          Chief Financial Officer      
David P. Southwell                    (Principal Financial Officer)

/s/ Robert F. Scumaci                 Senior Vice President,             March 26, 1998
----------------------------          Finance and Administration  
Robert F. Scumaci                     (Principal Accounting       
                                      Officer)                    

/s/James G. Andress                   Director                           March 26, 1998
----------------------------
James G. Andress

/s/ Digby W. Barrios                  Director                           March 26, 1998
----------------------------
Digby W. Barrios

/s/ Robert J. Cresci                  Director                           March 26, 1998
----------------------------
Robert J. Cresci

/s/ Robert F. Johnston                Director                           March 26, 1998
----------------------------
Robert F. Johnston
- 8 -
9
/s/ Keith Mansford                    Director                           March 26, 1998
----------------------------
Keith Mansford, Ph.D.

/s/ James F. Mrazek                   Director                           March 26, 1998
----------------------------
James F. Mrazek

/s/ Alan A. Steigrod                  Director                           March 26, 1998
----------------------------
Alan A. Steigrod
- 9 -
10

                                  EXHIBIT INDEX

Exhibit
Number         Description
------         -----------

  4.1*         Certificate of Incorporation of the Registrant, as amended

  4.2*         By-Laws of the Registrant

  4.3*         Specimen Certificate of Common Stock of the Registrant

  5            Opinion of Hale and Dorr LLP

 23.1          Consent of Hale and Dorr LLP (included in Exhibit 5)

 23.2          Consent of Coopers & Lybrand L.L.P.

 24            Power of Attorney (included in the signature pages of this
               Registration Statement)

------------------
*  Incorporated herein by reference from the Registrant's Registration
   Statement on Form S-1 (File No. 33-41653).

                                     - 10 -