<PAGE>

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 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):
                                   May 1, 2000





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




           Missouri                 1-14756                      43-1723446
(State or other jurisdiction      (Commission                 (I.R.S. Employer
        of incorporation)         File Number)               Identification No.)




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






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


<PAGE>


ITEM 5. OTHER EVENTS

     Reference  is made  to  Item 1.  Business  under  the  captions  "General",
"Capital  Program and Financing" and  "Regulation" and Item 2. Properties in the
Registrant's Form 10-K for the year ended December 31, 1999, and to Management's
Discussion and Analysis of Financial  Condition and Results of Operations  under
the caption  "Electric  Industry  Restructuring"  in the 1999  Annual  Report to
Stockholders pages incorporated by reference under Item 7 of the 1999 Form 10-K,
for a  discussion  of the  plan  of  Central  Illinois  Public  Service  Company
(AmerenCIPS),  the Registrant's subsidiary,  to transfer its electric generating
facilities  to a new  nonregulated  subsidiary  of the  Registrant  pursuant  to
Illinois' Electric Service Customer Choice and Rate Relief Law of 1997.

     On May 1, 2000,  following  the receipt of all  required  State and Federal
regulatory approvals, AmerenCIPS' transferred its electric generating assets and
liabilities to a newly created  nonregulated  company,  Ameren Energy Generating
Company (Generating  Company), a wholly-owned  subsidiary of the Registrant,  in
exchange for a promissory note from Generating  Company in the principal  amount
of approximately $600 million (the net book value of the assets and liabilities)
and  Generating  Company common stock.  The  promissory  note has a term of five
years and bears interest at 7% based on a 10-year amortization.  The transferred
assets  represent  a  generating  capacity  of  approximately  2,900  megawatts.
Approximately  45% of  AmerenCIPS'  employees  were  transferred  to  Generating
Company as a part of the transaction.

     Also on May 1, 2000,  an electric  power supply  agreement was entered into
between Generating Company and its newly created nonregulated affiliate,  Ameren
Energy  Marketing  Company  (Marketing  Company).  On the same  date,  Marketing
Company  entered into an electric  power supply  agreement  with  AmerenCIPS  to
supply it  sufficient  power to meet native load  requirements.  This  agreement
expires December 31, 2004. Power will continue to be jointly  dispatched between
Union Electric  Company,  another  subsidiary of the Registrant,  and Generating
Company.

     The  creation  of the new  subsidiaries  and the  transfer  of  AmerenCIPS'
generating  assets and liabilities had no effect on the financial  statements of
the Registrant as of the date of transfer.



                                    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
                                              (Registrant)


                                            By    /s/ Warner L. Baxter
                                                -------------------------
                                                      Warner L. Baxter
                                               Vice President and Controller
                                               (Principal Accounting Officer)


Date: May 5, 2000


                                      -2-
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UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 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):
                                   May 1, 2000

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

           Missouri                 1-14756                      43-1723446
(State or other jurisdiction      (Commission                 (I.R.S. Employer
        of incorporation)         File Number)               Identification No.)

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

       Registrant's telephone number, including area code: (314) 621-3222
ITEM 5. OTHER EVENTS

     Reference  is made  to  Item 1.  Business  under  the  captions  "General",
"Capital  Program and Financing" and  "Regulation" and Item 2. Properties in the
Registrant's Form 10-K for the year ended December 31, 1999, and to Management's
Discussion and Analysis of Financial  Condition and Results of Operations  under
the caption  "Electric  Industry  Restructuring"  in the 1999  Annual  Report to
Stockholders pages incorporated by reference under Item 7 of the 1999 Form 10-K,
for a  discussion  of the  plan  of  Central  Illinois  Public  Service  Company
(AmerenCIPS),  the Registrant's subsidiary,  to transfer its electric generating
facilities  to a new  nonregulated  subsidiary  of the  Registrant  pursuant  to
Illinois' Electric Service Customer Choice and Rate Relief Law of 1997.

     On May 1, 2000,  following  the receipt of all  required  State and Federal
regulatory approvals, AmerenCIPS' transferred its electric generating assets and
liabilities to a newly created  nonregulated  company,  Ameren Energy Generating
Company (Generating  Company), a wholly-owned  subsidiary of the Registrant,  in
exchange for a promissory note from Generating  Company in the principal  amount
of approximately $600 million (the net book value of the assets and liabilities)
and  Generating  Company common stock.  The  promissory  note has a term of five
years and bears interest at 7% based on a 10-year amortization.  The transferred
assets  represent  a  generating  capacity  of  approximately  2,900  megawatts.
Approximately  45% of  AmerenCIPS'  employees  were  transferred  to  Generating
Company as a part of the transaction.

     Also on May 1, 2000,  an electric  power supply  agreement was entered into
between Generating Company and its newly created nonregulated affiliate,  Ameren
Energy  Marketing  Company  (Marketing  Company).  On the same  date,  Marketing
Company  entered into an electric  power supply  agreement  with  AmerenCIPS  to
supply it  sufficient  power to meet native load  requirements.  This  agreement
expires December 31, 2004. Power will continue to be jointly  dispatched between
Union Electric  Company,  another  subsidiary of the Registrant,  and Generating
Company.

     The  creation  of the new  subsidiaries  and the  transfer  of  AmerenCIPS'
generating  assets and liabilities had no effect on the financial  statements of
the Registrant as of the date of transfer.

                                    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
                                              (Registrant)

                                            By    /s/ Warner L. Baxter
                                                -------------------------
                                                      Warner L. Baxter
                                               Vice President and Controller
                                               (Principal Accounting Officer)

Date: May 5, 2000

                                      -2-