<PAGE>
                                                                                

                       SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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

                                    FORM 8-K


                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 or 15(d) of the
                         SECURITIES EXCHANGE ACT OF 1934


Date of Report (Date of earliest event reported):  October 9, 1998



                               AMEREN CORPORATION
-------------------------------------------------------------------------------
               (Exact name of registrant as specified in charter)




                                    Missouri
-------------------------------------------------------------------------------
                           State or other jurisdiction
                                of incorporation)



      1-14756                                                    43-1723446
      -------                                                    ----------
(Commission File No.)                                           (IRS employer
                                                             identification no.)



1901 Chouteau Avenue, St. Louis, Missouri                            63103
--------------------------------------------------------------------------------
(Address of principal executive offices)                           (Zip Code)



Registrant's telephone number, including area code (314) 621-3222
                                                   --------------


<PAGE>


                    INFORMATION TO BE INCLUDED IN THE REPORT

Item 5.  Other Events.

Introduction
------------

                  On  October  9,  1998,   the  Board  of  Directors  of  Ameren
Corporation (the "Company")  declared a dividend of one preferred share purchase
right (a "Right") for each outstanding share of common stock, par value $.01 per
share,  of the Company  (the  "Common  Shares").  The dividend is payable on the
tenth day following  approval by the Securities  and Exchange  Commission of the
issuance of the Rights under the Public Utility  Holding Company Act of 1935, as
amended (such date,  the "Record  Date") to the  stockholders  of record on that
date. The description and terms of the Rights are set forth in an Agreement (the
"Agreement") between the Company and First Chicago Trust Company of New York, as
Rights Agent (the "Rights Agent").

Purchase Price
--------------

                  Each Right entitles the registered holder to purchase from the
Company one one-hundredth of a share of Series A Junior Participating  Preferred
Stock of the Company,  par value $.01 per share (the "Preferred  Shares"),  at a
price of $180 per one one-hundredth of a Preferred Share (the "Purchase Price"),
subject to adjustment.

Flip-In
-------

                  In the  event  that  any  person  or group  of  affiliated  or
associated  persons  acquires  beneficial  ownership  of  15%  or  more  of  the
outstanding Common Shares (an "Acquiring Person"), each holder of a Right, other
than Rights beneficially owned by the Acquiring Person (which will thereafter be
void),  will  thereafter  have the right to receive upon exercise that number of
Common  Shares  having a market  value of two  times the  exercise  price of the
Right.

Flip-Over
---------

                  If the  Company  is  acquired  in a merger  or other  business
combination  transaction  or 50% or more of its  consolidated  assets or earning
power are sold  after a person or group has  become an  Acquiring  Person,  each
holder of a Right (other than Rights  beneficially  owned by  Acquiring  Person,
which will be void) will  thereafter  have the right to receive  that  number of
shares  of  common  stock  of the  acquiring  company  which at the time of such
transaction  will have a market  value of two times  the  exercise  price of the
Right.

                                      -1-

<PAGE>

Distribution Date
-----------------

                  The distribution date is the earlier of

                  (i) 10 days following a public  announcement  that a person or
group of affiliated or associated persons have acquired beneficial  ownership of
15% or more of the outstanding Common Shares; or

                  (ii) 10 business days (or such later date as may be determined
by action of the Board of  Directors  of the  Company  prior to such time as any
person or group of affiliated persons becomes an Acquiring Person) following the
commencement  of, or  announcement  of an  intention  to make, a tender offer or
exchange  offer  the  consummation  of  which  would  result  in the  beneficial
ownership by a person or group of 15% or more of the outstanding Common Shares.

Transfer and Detachment
-----------------------

                  Until the  Distribution  Date,  the Rights will be  evidenced,
with  respect  to any of the Common  Share  certificates  outstanding  as of the
Record  Date,  by such Common Share  certificate  with a copy of this Summary of
Rights attached thereto.  Until the Distribution Date (or earlier  redemption or
expiration of the Rights), the Rights will be transferred with and only with the
Common Shares, and transfer of those certificates will also constitute  transfer
of these Rights.

                  As  soon  as  practicable  following  the  Distribution  Date,
separate  certificates  evidencing  the Rights  ("Right  Certificates")  will be
mailed to holders of record of the Common  Shares as of the close of business on
the Distribution Date and such separate Right Certificates alone will thereafter
evidence the Rights.

Exercisability
--------------
                  The Rights are not exercisable  until the  Distribution  Date.
The Rights will expire on October 9, 2008 (the "Final Expiration Date"),  unless
the Final  Expiration Date is extended or unless the Rights are earlier redeemed
or exchanged by the Company, in each case, as described below.

Adjustments
-----------
                  The Purchase Price payable, and the number of Preferred Shares
or other  securities  or  property  issuable,  upon  exercise  of the Rights are
subject  to  adjustment  from time to time to prevent  dilution  in the event of
stock dividends, stock splits, reclassifications,  or certain distributions with
respect to the Preferred Shares. The number of outstanding Rights and the number
of one  one-hundredths of a Preferred Share issuable upon exercise of each Right
are also subject to adjustment if, prior to the  Distribution  Date,  there is a
stock  split of the  Common  Shares or a stock  dividend  on the  Common  Shares
payable in Common Shares or subdivisions,  consolidations or combinations of the
Common Shares. With certain exceptions, no adjustment in the Purchase Price will
be required until cumulative adjustments require an adjustment of at least 1% in
such Purchase Price. No fractional  Preferred  Shares will be issued (other than
fractions which

                                      -2-
<PAGE>

are integral multiples of one one-hundredth of a Preferred Share,  which may, at
the  election of the Company, be evidenced by  depositary receipts) and, in lieu
thereof,  an  adjustment in  cash will be made based on the  market price of the
Preferred Shares on the last trading day prior to the date of exercise.

Preferred Shares
----------------

                  Preferred Shares  purchasable upon exercise of the Rights will
not  be  redeemable.  Each  Preferred  Share  will  be  entitled  to  a  minimum
preferential  quarterly dividend payment of $1 per share but will be entitled to
an aggregate  dividend of 100 times the dividend  declared per Common Share.  In
the event of liquidation,  the holders of the Preferred  Shares will be entitled
to a  minimum  preferential  liquidation  payment  of $100 per share but will be
entitled to an aggregate payment of 100 times the payment made per Common Share.
Each  Preferred  Share  will have 100  votes,  voting  together  with the Common
Shares. Finally, in the event of any merger,  consolidation or other transaction
in which Common Shares are exchanged,  each Preferred  Share will be entitled to
receive  100 times the  amount  received  per  Common  Share.  These  rights are
protected by customary antidilution provisions.

                  The value of the one  one-hundredth  interest  in a  Preferred
Share  purchasable upon exercise of each Right should,  because of the nature of
the Preferred Shares' dividend,  liquidation and voting rights,  approximate the
value of one Common Share.

Exchange
--------

                  At any time  after any person or group  becomes  an  Acquiring
Person,  and prior to the  acquisition by such person or group of 50% or more of
the  outstanding  Common  Shares,  the Board of  Directors  of the  Company  may
exchange the Rights (other than Rights owned by the Acquiring Person, which will
have  become  void),  in whole or in part,  at an  exchange  ratio of one Common
Share, or one one-hundredth of a Preferred Share (subject to adjustment).

Redemption
----------

                  At any time prior to any person or group becoming an Acquiring
Person,  the Board of  Directors  of the Company may redeem the Rights in whole,
but not in part,  at a price of $.01 per Right  (the  "Redemption  Price").  The
redemption  of the Rights may be made  effective at such time on such basis with
such  conditions as the Board of Directors in its sole discretion may establish.
Immediately upon any redemption of the Rights,  the right to exercise the Rights
will  terminate  and the only right of the  holders of Rights will be to receive
the Redemption Price.

Amendments
----------

                  The  terms  of the  Rights  may be  amended  by the  Board  of
Directors  of the  Company  without  the  consent of the  holders of the Rights,
including an amendment to lower certain  thresholds  described above to not less
than the  greater  of (i) the sum of .001%  and the  largest  percentage  of the
outstanding  Common Shares then known to the Company to be beneficially owned by
any person or group of  affiliated or  associated  persons and (ii) 10%,  except
that from
                                      -3-
<PAGE>

and after such time as any  person or group  of affiliated or associated persons
becomes an Acquiring Person no such amendment may adversely affect the interests
of the holders of the Rights.

Rights and Holders
------------------

                  Until a Right is exercised,  the holder thereof, as such, will
have no rights as a stockholder of the Company,  including,  without limitation,
the right to vote or to receive dividends.

Further Information
-------------------

                  A copy of the Agreement  and the related press release  issued
by the  Company on October 9, 1998 are filed as Exhibits 4 and 99 hereto and are
incorporated  herein by reference.  The  foregoing  summary  description  of the
Rights does not  purport to be  complete  and is  qualified  in its  entirety by
reference to the Agreement.

Item 7.  Financial Statements, Pro Forma Financial Information and Exhibits.
                               --- -----

                  (c)      Exhibits

                           4        Agreement,  dated  as of  October  9,  1998,
                                    between Ameren Corporation and First Chicago
                                    Trust  Company of New York, as Rights Agent,
                                    which  includes the form of  Certificate  of
                                    Designation  of  the  Preferred   Shares  as
                                    Exhibit A, the form of Right  Certificate as
                                    Exhibit  B and  the  Summary  of  Rights  as
                                    Exhibit C.

                           99       Press Release  dated October 9, 1998, issued
                                    by Ameren Corporation.



                                      -4-

<PAGE>


                                    SIGNATURE

                  Pursuant to the requirements of the Securities Exchange Act of
1934,  the  registrant has duly caused this report to be signed on its behalf by
the undersigned thereunto duly authorized.

                                                   AMEREN CORPORATION



                                       By:   /S/  Steven R. Sullivan
                                           ----------------------------------
                                      Name: Steven R. Sullivan
                                     Title: Vice President, General Counsel
                                                  and Secretary




Dated:  October 14, 1998


                                      -5-

<PAGE>


                                  EXHIBIT INDEX

Exhibit                                                           
  No.                              Description                    
-------                            -----------                    

  4               Agreement, dated as of October 9, 1998,
                  between  Ameren  Corporation  and First
                  Chicago  Trust Company of  New York, as
                  Rights Agent, which includes  the  form
                  of  Certificate  of  Designation of the
                  Preferred Shares as Exhibit A, the form
                  of Right  Certificate  as Exhibit B and
                  the  Summary of Rights as Exhibit C.

  99              Press  Release  dated  October 9,  1998,
                  issued  by Ameren Corporation.




_____________________________


                                      -6-
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SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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

                                    FORM 8-K

                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 or 15(d) of the
                         SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported):  October 9, 1998

                               AMEREN CORPORATION
-------------------------------------------------------------------------------
               (Exact name of registrant as specified in charter)

                                    Missouri
-------------------------------------------------------------------------------
                           State or other jurisdiction
                                of incorporation)

      1-14756                                                    43-1723446
      -------                                                    ----------
(Commission File No.)                                           (IRS employer
                                                             identification no.)

1901 Chouteau Avenue, St. Louis, Missouri                            63103
--------------------------------------------------------------------------------
(Address of principal executive offices)                           (Zip Code)

Registrant's telephone number, including area code (314) 621-3222
                                                   --------------
INFORMATION TO BE INCLUDED IN THE REPORT

Item 5.  Other Events.

Introduction
------------

                  On  October  9,  1998,   the  Board  of  Directors  of  Ameren
Corporation (the "Company")  declared a dividend of one preferred share purchase
right (a "Right") for each outstanding share of common stock, par value $.01 per
share,  of the Company  (the  "Common  Shares").  The dividend is payable on the
tenth day following  approval by the Securities  and Exchange  Commission of the
issuance of the Rights under the Public Utility  Holding Company Act of 1935, as
amended (such date,  the "Record  Date") to the  stockholders  of record on that
date. The description and terms of the Rights are set forth in an Agreement (the
"Agreement") between the Company and First Chicago Trust Company of New York, as
Rights Agent (the "Rights Agent").

Purchase Price
--------------

                  Each Right entitles the registered holder to purchase from the
Company one one-hundredth of a share of Series A Junior Participating  Preferred
Stock of the Company,  par value $.01 per share (the "Preferred  Shares"),  at a
price of $180 per one one-hundredth of a Preferred Share (the "Purchase Price"),
subject to adjustment.

Flip-In
-------

                  In the  event  that  any  person  or group  of  affiliated  or
associated  persons  acquires  beneficial  ownership  of  15%  or  more  of  the
outstanding Common Shares (an "Acquiring Person"), each holder of a Right, other
than Rights beneficially owned by the Acquiring Person (which will thereafter be
void),  will  thereafter  have the right to receive upon exercise that number of
Common  Shares  having a market  value of two  times the  exercise  price of the
Right.

Flip-Over
---------

                  If the  Company  is  acquired  in a merger  or other  business
combination  transaction  or 50% or more of its  consolidated  assets or earning
power are sold  after a person or group has  become an  Acquiring  Person,  each
holder of a Right (other than Rights  beneficially  owned by  Acquiring  Person,
which will be void) will  thereafter  have the right to receive  that  number of
shares  of  common  stock  of the  acquiring  company  which at the time of such
transaction  will have a market  value of two times  the  exercise  price of the
Right.

                                      -1-
Distribution Date
-----------------

                  The distribution date is the earlier of

                  (i) 10 days following a public  announcement  that a person or
group of affiliated or associated persons have acquired beneficial  ownership of
15% or more of the outstanding Common Shares; or

                  (ii) 10 business days (or such later date as may be determined
by action of the Board of  Directors  of the  Company  prior to such time as any
person or group of affiliated persons becomes an Acquiring Person) following the
commencement  of, or  announcement  of an  intention  to make, a tender offer or
exchange  offer  the  consummation  of  which  would  result  in the  beneficial
ownership by a person or group of 15% or more of the outstanding Common Shares.

Transfer and Detachment
-----------------------

                  Until the  Distribution  Date,  the Rights will be  evidenced,
with  respect  to any of the Common  Share  certificates  outstanding  as of the
Record  Date,  by such Common Share  certificate  with a copy of this Summary of
Rights attached thereto.  Until the Distribution Date (or earlier  redemption or
expiration of the Rights), the Rights will be transferred with and only with the
Common Shares, and transfer of those certificates will also constitute  transfer
of these Rights.

                  As  soon  as  practicable  following  the  Distribution  Date,
separate  certificates  evidencing  the Rights  ("Right  Certificates")  will be
mailed to holders of record of the Common  Shares as of the close of business on
the Distribution Date and such separate Right Certificates alone will thereafter
evidence the Rights.

Exercisability
--------------
                  The Rights are not exercisable  until the  Distribution  Date.
The Rights will expire on October 9, 2008 (the "Final Expiration Date"),  unless
the Final  Expiration Date is extended or unless the Rights are earlier redeemed
or exchanged by the Company, in each case, as described below.

Adjustments
-----------
                  The Purchase Price payable, and the number of Preferred Shares
or other  securities  or  property  issuable,  upon  exercise  of the Rights are
subject  to  adjustment  from time to time to prevent  dilution  in the event of
stock dividends, stock splits, reclassifications,  or certain distributions with
respect to the Preferred Shares. The number of outstanding Rights and the number
of one  one-hundredths of a Preferred Share issuable upon exercise of each Right
are also subject to adjustment if, prior to the  Distribution  Date,  there is a
stock  split of the  Common  Shares or a stock  dividend  on the  Common  Shares
payable in Common Shares or subdivisions,  consolidations or combinations of the
Common Shares. With certain exceptions, no adjustment in the Purchase Price will
be required until cumulative adjustments require an adjustment of at least 1% in
such Purchase Price. No fractional  Preferred  Shares will be issued (other than
fractions which

                                      -2-
are integral multiples of one one-hundredth of a Preferred Share,  which may, at
the  election of the Company, be evidenced by  depositary receipts) and, in lieu
thereof,  an  adjustment in  cash will be made based on the  market price of the
Preferred Shares on the last trading day prior to the date of exercise.

Preferred Shares
----------------

                  Preferred Shares  purchasable upon exercise of the Rights will
not  be  redeemable.  Each  Preferred  Share  will  be  entitled  to  a  minimum
preferential  quarterly dividend payment of $1 per share but will be entitled to
an aggregate  dividend of 100 times the dividend  declared per Common Share.  In
the event of liquidation,  the holders of the Preferred  Shares will be entitled
to a  minimum  preferential  liquidation  payment  of $100 per share but will be
entitled to an aggregate payment of 100 times the payment made per Common Share.
Each  Preferred  Share  will have 100  votes,  voting  together  with the Common
Shares. Finally, in the event of any merger,  consolidation or other transaction
in which Common Shares are exchanged,  each Preferred  Share will be entitled to
receive  100 times the  amount  received  per  Common  Share.  These  rights are
protected by customary antidilution provisions.

                  The value of the one  one-hundredth  interest  in a  Preferred
Share  purchasable upon exercise of each Right should,  because of the nature of
the Preferred Shares' dividend,  liquidation and voting rights,  approximate the
value of one Common Share.

Exchange
--------

                  At any time  after any person or group  becomes  an  Acquiring
Person,  and prior to the  acquisition by such person or group of 50% or more of
the  outstanding  Common  Shares,  the Board of  Directors  of the  Company  may
exchange the Rights (other than Rights owned by the Acquiring Person, which will
have  become  void),  in whole or in part,  at an  exchange  ratio of one Common
Share, or one one-hundredth of a Preferred Share (subject to adjustment).

Redemption
----------

                  At any time prior to any person or group becoming an Acquiring
Person,  the Board of  Directors  of the Company may redeem the Rights in whole,
but not in part,  at a price of $.01 per Right  (the  "Redemption  Price").  The
redemption  of the Rights may be made  effective at such time on such basis with
such  conditions as the Board of Directors in its sole discretion may establish.
Immediately upon any redemption of the Rights,  the right to exercise the Rights
will  terminate  and the only right of the  holders of Rights will be to receive
the Redemption Price.

Amendments
----------

                  The  terms  of the  Rights  may be  amended  by the  Board  of
Directors  of the  Company  without  the  consent of the  holders of the Rights,
including an amendment to lower certain  thresholds  described above to not less
than the  greater  of (i) the sum of .001%  and the  largest  percentage  of the
outstanding  Common Shares then known to the Company to be beneficially owned by
any person or group of  affiliated or  associated  persons and (ii) 10%,  except
that from
                                      -3-
and after such time as any  person or group  of affiliated or associated persons
becomes an Acquiring Person no such amendment may adversely affect the interests
of the holders of the Rights.

Rights and Holders
------------------

                  Until a Right is exercised,  the holder thereof, as such, will
have no rights as a stockholder of the Company,  including,  without limitation,
the right to vote or to receive dividends.

Further Information
-------------------

                  A copy of the Agreement  and the related press release  issued
by the  Company on October 9, 1998 are filed as Exhibits 4 and 99 hereto and are
incorporated  herein by reference.  The  foregoing  summary  description  of the
Rights does not  purport to be  complete  and is  qualified  in its  entirety by
reference to the Agreement.

Item 7.  Financial Statements, Pro Forma Financial Information and Exhibits.
                               --- -----

                  (c)      Exhibits

                           4        Agreement,  dated  as of  October  9,  1998,
                                    between Ameren Corporation and First Chicago
                                    Trust  Company of New York, as Rights Agent,
                                    which  includes the form of  Certificate  of
                                    Designation  of  the  Preferred   Shares  as
                                    Exhibit A, the form of Right  Certificate as
                                    Exhibit  B and  the  Summary  of  Rights  as
                                    Exhibit C.

                           99       Press Release  dated October 9, 1998, issued
                                    by Ameren Corporation.

                                      -4-
SIGNATURE

                  Pursuant to the requirements of the Securities Exchange Act of
1934,  the  registrant has duly caused this report to be signed on its behalf by
the undersigned thereunto duly authorized.

                                                   AMEREN CORPORATION

                                       By:   /S/  Steven R. Sullivan
                                           ----------------------------------
                                      Name: Steven R. Sullivan
                                     Title: Vice President, General Counsel
                                                  and Secretary

Dated:  October 14, 1998

                                      -5-
EXHIBIT INDEX

Exhibit                                                           
  No.                              Description                    
-------                            -----------                    

  4               Agreement, dated as of October 9, 1998,
                  between  Ameren  Corporation  and First
                  Chicago  Trust Company of  New York, as
                  Rights Agent, which includes  the  form
                  of  Certificate  of  Designation of the
                  Preferred Shares as Exhibit A, the form
                  of Right  Certificate  as Exhibit B and
                  the  Summary of Rights as Exhibit C.

  99              Press  Release  dated  October 9,  1998,
                  issued  by Ameren Corporation.

_____________________________

                                      -6-