<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):
                                December 23, 1999





                               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  Note 5 - under  Notes  to  Consolidated  Financial
Statements in the  Registrant's  Form 10-Q for the quarter  ended  September 30,
1999, for a discussion of the regulatory  proceedings before the Missouri Public
Service  Commission  (MoPSC) involving the experimental  alternative  regulation
plans approved by that agency with respect to the retail electric rates of Union
Electric Company (AmerenUE),  a subsidiary of the Registrant.  In July 1995, the
MoPSC approved an agreement  among  AmerenUE,  the MoPSC Staff and other parties
which  included  a  three-year  experimental  alternative  regulation  plan (the
Original  Plan) that ran from July 1, 1995 through  June 30, 1998.  The Original
Plan  provided  that  earnings in those  years in excess of a 12.61%  regulatory
return on equity (ROE) be shared equally between  AmerenUE's  electric customers
and  shareholders,  and earnings  above a 14% ROE be credited to customers.  The
formula for  computing  the credit  used  twelve-month  results  ending June 30,
rather than calendar year earnings.  In 1998, AmerenUE recorded an estimated $43
million credit for the final year of the Original Plan.

     Included in the joint agreement  approved by the MoPSC in its February 1997
order authorizing the merger between AmerenUE and CIPSCO  Incorporated was a new
three-year  experimental  alternative  regulation  plan (the New Plan) that runs
from July 1, 1998 through June 30, 2001.  Like the Original  Plan,  the New Plan
requires  that  earnings  over a 12.61%  ROE up to a 14% ROE be  shared  equally
between  electric  customers  and  shareholders.  The New Plan also  returns  to
customers 90% of all earnings above a 14% ROE up to a 16% ROE.  Earnings above a
16% ROE are to be  credited  entirely  to  customers.  In  addition,  the  joint
agreement  provides  for a  Missouri  electric  rate  decrease,  retroactive  to
September  1, 1998,  based on the  weather-adjusted  average  annual  credits to
customers under the Original Plan. AmerenUE estimated that its Missouri electric
rate  decrease  would  approximate  $20 million on an  annualized  basis and has
reduced revenues accordingly since September 1998.

     In  November  1998,  the MoPSC Staff  proposed  adjustments  to  AmerenUE's
estimated credit for the third year of the Original Plan. In addition, the MoPSC
Staff  proposed  adjustments  to  AmerenUE's  estimated  Missouri  electric rate
decrease  based upon  their  methodology  of  calculating  the  weather-adjusted
credits.  The  determination  of the credit  for the third year of the  Original
Plan, as well as the determination of the Missouri electric rate decrease,  were
subject to regulatory proceedings before the MoPSC in 1999.

     On December 23, 1999, the MoPSC issued a Report and Order (Order)  relating
to the customer  credit for the third year of the Original Plan.  Certain of the
MoPSC Staff's  proposed  adjustments were accepted by the MoPSC in the Order. In
addition,  the Order requires  AmerenUE to capitalize and amortize certain costs
(including  computer  software cost) that had  previously  been expensed for its
Missouri  electric   operations.   On  December  30,  1999,  AmerenUE  filed  an
application  for  rehearing  of the Order  requesting  the MoPSC to reverse  its
decision.

     Based on the  provisions of the Order,  AmerenUE  estimates that the credit
for the  third  year of the  Original  Plan will  approximate  $31  million.  In
addition,  with regard to the Missouri  electric rate  decrease,  AmerenUE,  the
MoPSC Staff and other parties  reached a settlement  relating to the calculation
of the  weather-adjusted  credits.  As a  result,  AmerenUE  estimates  that the
annualized Missouri electric rate decrease will approximate $17 million. Both of
these estimates are subject to further approval by the MoPSC.

     The provisions of the Order also have an impact on the estimated  credit to
electric customers recorded by AmerenUE for the first year of the New Plan. As a
result, AmerenUE increased its estimated credit for the New Plan year ended June
30, 1999,  from $20 million to $25 million.  In addition,  AmerenUE  recorded an
estimated  $20  million  credit for the second year of the New Plan.  Also,  the
provision  of the Order which  requires  AmerenUE  to  capitalize  and  amortize
certain  costs  (including  computer  software  costs) that had been  previously
expensed,  resulted in the  capitalization of approximately $20 million of costs
in the fourth quarter of 1999.

<PAGE>

     In  summary,  the  provisions  of the Order and the  resulting  changes  in
AmerenUE's  estimates  of the  customer  credits  for the open  years  under the
Original Plan and the New Plan and of the electric rate decrease  resulted in an
increase  in  earnings of  approximately  $9  million,  or $.06 per share in the
fourth quarter of 1999.


SAFE HARBOR STATEMENT

Statements made in this Current Report which are not based on historical  facts,
are  "forward-looking"  and,  accordingly,  involve risks and uncertainties that
could cause actual results to differ  materially from those discussed.  Although
such "forward-looking"  statements have been made in good faith and are based on
reasonable assumptions,  there is no assurance that the expected results will be
achieved.  These statements include (without limitation) statements as to future
expectations,  beliefs, plans, strategies,  objectives,  events, conditions, and
financial  performance.  In connection with the "Safe Harbor"  provisions of the
Private  Securities  Litigation  Reform Act of 1995, the Registrant is providing
this cautionary  statement to identify important factors that could cause actual
results to differ materially from those anticipated.  The following factors,  in
addition  to those  discussed  elsewhere  in this  report and in the 1998 Annual
Report to Stockholders  (which is incorporated by reference in the  Registrant's
1998 Form 10-K) and in  subsequent  securities  filings,  could cause results to
differ   materially   from   management   expectations   as  suggested  by  such
"forward-looking" statements: the effects of regulatory actions; changes in laws
and other  governmental  actions;  business  and  economic  conditions;  weather
conditions; and legal and administrative proceedings.



                                    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:  January 20, 2000
[{"type": "8-K", "sequence": "1", "description": "FORM 8-K FOR AMEREN CORPORATION", "secsgml_size_bytes": 8249}]
{"company-data": {"conformed-name": "AMEREN CORP", "cik": "0001002910", "assigned-sic": "4931", "irs-number": "431723446", "state-of-incorporation": "MO", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "8-K", "act": "34", "file-number": "001-14756", "film-number": "510201"}, "business-address": {"street1": "1901 CHOUTEAU AVE", "city": "ST LOUIS", "state": "MO", "zip": "63103", "phone": "3146213222"}, "mail-address": {"street1": "1901 CHOUTEAU AVE", "city": "ST LOUIS", "state": "MO", "zip": "63103"}}
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):
                                December 23, 1999

                               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  Note 5 - under  Notes  to  Consolidated  Financial
Statements in the  Registrant's  Form 10-Q for the quarter  ended  September 30,
1999, for a discussion of the regulatory  proceedings before the Missouri Public
Service  Commission  (MoPSC) involving the experimental  alternative  regulation
plans approved by that agency with respect to the retail electric rates of Union
Electric Company (AmerenUE),  a subsidiary of the Registrant.  In July 1995, the
MoPSC approved an agreement  among  AmerenUE,  the MoPSC Staff and other parties
which  included  a  three-year  experimental  alternative  regulation  plan (the
Original  Plan) that ran from July 1, 1995 through  June 30, 1998.  The Original
Plan  provided  that  earnings in those  years in excess of a 12.61%  regulatory
return on equity (ROE) be shared equally between  AmerenUE's  electric customers
and  shareholders,  and earnings  above a 14% ROE be credited to customers.  The
formula for  computing  the credit  used  twelve-month  results  ending June 30,
rather than calendar year earnings.  In 1998, AmerenUE recorded an estimated $43
million credit for the final year of the Original Plan.

     Included in the joint agreement  approved by the MoPSC in its February 1997
order authorizing the merger between AmerenUE and CIPSCO  Incorporated was a new
three-year  experimental  alternative  regulation  plan (the New Plan) that runs
from July 1, 1998 through June 30, 2001.  Like the Original  Plan,  the New Plan
requires  that  earnings  over a 12.61%  ROE up to a 14% ROE be  shared  equally
between  electric  customers  and  shareholders.  The New Plan also  returns  to
customers 90% of all earnings above a 14% ROE up to a 16% ROE.  Earnings above a
16% ROE are to be  credited  entirely  to  customers.  In  addition,  the  joint
agreement  provides  for a  Missouri  electric  rate  decrease,  retroactive  to
September  1, 1998,  based on the  weather-adjusted  average  annual  credits to
customers under the Original Plan. AmerenUE estimated that its Missouri electric
rate  decrease  would  approximate  $20 million on an  annualized  basis and has
reduced revenues accordingly since September 1998.

     In  November  1998,  the MoPSC Staff  proposed  adjustments  to  AmerenUE's
estimated credit for the third year of the Original Plan. In addition, the MoPSC
Staff  proposed  adjustments  to  AmerenUE's  estimated  Missouri  electric rate
decrease  based upon  their  methodology  of  calculating  the  weather-adjusted
credits.  The  determination  of the credit  for the third year of the  Original
Plan, as well as the determination of the Missouri electric rate decrease,  were
subject to regulatory proceedings before the MoPSC in 1999.

     On December 23, 1999, the MoPSC issued a Report and Order (Order)  relating
to the customer  credit for the third year of the Original Plan.  Certain of the
MoPSC Staff's  proposed  adjustments were accepted by the MoPSC in the Order. In
addition,  the Order requires  AmerenUE to capitalize and amortize certain costs
(including  computer  software cost) that had  previously  been expensed for its
Missouri  electric   operations.   On  December  30,  1999,  AmerenUE  filed  an
application  for  rehearing  of the Order  requesting  the MoPSC to reverse  its
decision.

     Based on the  provisions of the Order,  AmerenUE  estimates that the credit
for the  third  year of the  Original  Plan will  approximate  $31  million.  In
addition,  with regard to the Missouri  electric rate  decrease,  AmerenUE,  the
MoPSC Staff and other parties  reached a settlement  relating to the calculation
of the  weather-adjusted  credits.  As a  result,  AmerenUE  estimates  that the
annualized Missouri electric rate decrease will approximate $17 million. Both of
these estimates are subject to further approval by the MoPSC.

     The provisions of the Order also have an impact on the estimated  credit to
electric customers recorded by AmerenUE for the first year of the New Plan. As a
result, AmerenUE increased its estimated credit for the New Plan year ended June
30, 1999,  from $20 million to $25 million.  In addition,  AmerenUE  recorded an
estimated  $20  million  credit for the second year of the New Plan.  Also,  the
provision  of the Order which  requires  AmerenUE  to  capitalize  and  amortize
certain  costs  (including  computer  software  costs) that had been  previously
expensed,  resulted in the  capitalization of approximately $20 million of costs
in the fourth quarter of 1999.
In  summary,  the  provisions  of the Order and the  resulting  changes  in
AmerenUE's  estimates  of the  customer  credits  for the open  years  under the
Original Plan and the New Plan and of the electric rate decrease  resulted in an
increase  in  earnings of  approximately  $9  million,  or $.06 per share in the
fourth quarter of 1999.

SAFE HARBOR STATEMENT

Statements made in this Current Report which are not based on historical  facts,
are  "forward-looking"  and,  accordingly,  involve risks and uncertainties that
could cause actual results to differ  materially from those discussed.  Although
such "forward-looking"  statements have been made in good faith and are based on
reasonable assumptions,  there is no assurance that the expected results will be
achieved.  These statements include (without limitation) statements as to future
expectations,  beliefs, plans, strategies,  objectives,  events, conditions, and
financial  performance.  In connection with the "Safe Harbor"  provisions of the
Private  Securities  Litigation  Reform Act of 1995, the Registrant is providing
this cautionary  statement to identify important factors that could cause actual
results to differ materially from those anticipated.  The following factors,  in
addition  to those  discussed  elsewhere  in this  report and in the 1998 Annual
Report to Stockholders  (which is incorporated by reference in the  Registrant's
1998 Form 10-K) and in  subsequent  securities  filings,  could cause results to
differ   materially   from   management   expectations   as  suggested  by  such
"forward-looking" statements: the effects of regulatory actions; changes in laws
and other  governmental  actions;  business  and  economic  conditions;  weather
conditions; and legal and administrative proceedings.

                                    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:  January 20, 2000