<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





                             UNION ELECTRIC COMPANY
             (Exact name of registrant as specified in its charter)




           Missouri                   1-2967                     43-0559760
(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 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 the
Registrant, a subsidiary of Ameren Corporation. In July 1995, the MoPSC approved
an  agreement  among the  Registrant,  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 the Registrant'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 the Registrant 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.  The Registrant  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 the Registrant's
estimated credit for the third year of the Original Plan. In addition, the MoPSC
Staff proposed adjustments to the Registrant'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 the Registrant to capitalize and amortize  certain
costs (including  computer software costs) that had previously been expensed for
its Missouri electric operations.  On December 30, 1999, the Registrant filed an
application  for  rehearing  of the Order  requesting  the MoPSC to reverse  its
decision.

     Based on the  provisions of the Order,  the  Registrant  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,  the Registrant,
the  MoPSC  Staff  and  other  parties  reached  a  settlement  relating  to the
calculation  of  the  weather-adjusted  credits.  As a  result,  the  Registrant
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 the  Registrant  for the first year of the New
Plan. As a result,  the  Registrant  increased its estimated  credit for the New
Plan year ended June 30, 1999, from $20 million to $25 million. In addition, the
Registrant  recorded an estimated $20 million  credit for the second year of the
New Plan.  Also,  the  provision of the Order which  requires the  Registrant 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 the
Registrant'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.

                                                UNION ELECTRIC COMPANY
                                                    (Registrant)


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


Date:  January 20, 2000
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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):
                                December 23, 1999

                             UNION ELECTRIC COMPANY
             (Exact name of registrant as specified in its charter)

           Missouri                   1-2967                     43-0559760
(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 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 the
Registrant, a subsidiary of Ameren Corporation. In July 1995, the MoPSC approved
an  agreement  among the  Registrant,  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 the Registrant'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 the Registrant 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.  The Registrant  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 the Registrant's
estimated credit for the third year of the Original Plan. In addition, the MoPSC
Staff proposed adjustments to the Registrant'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 the Registrant to capitalize and amortize  certain
costs (including  computer software costs) that had previously been expensed for
its Missouri electric operations.  On December 30, 1999, the Registrant filed an
application  for  rehearing  of the Order  requesting  the MoPSC to reverse  its
decision.

     Based on the  provisions of the Order,  the  Registrant  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,  the Registrant,
the  MoPSC  Staff  and  other  parties  reached  a  settlement  relating  to the
calculation  of  the  weather-adjusted  credits.  As a  result,  the  Registrant
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 the  Registrant  for the first year of the New
Plan. As a result,  the  Registrant  increased its estimated  credit for the New
Plan year ended June 30, 1999, from $20 million to $25 million. In addition, the
Registrant  recorded an estimated $20 million  credit for the second year of the
New Plan.  Also,  the  provision of the Order which  requires the  Registrant 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 the
Registrant'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.

                                                UNION ELECTRIC COMPANY
                                                    (Registrant)

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

Date:  January 20, 2000