<PAGE>

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549


                                    FORM 10-Q

[X]  QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE
     ACT OF 1934

     For Quarterly Period Ended September 30, 1999

[ ]  TRANSITION  REPORT  PURSUANT  TO  SECTION 13 OR  15(d)  OF  THE  SECURITIES
     EXCHANGE ACT OF 1934

     For The Transition Period From                  to

                         Commission file number 1-3672.

                     CENTRAL ILLINOIS PUBLIC SERVICE COMPANY
             (Exact name of registrant as specified in its charter)

            Illinois                                             37-0211380
(State or other jurisdiction of                               (I.R.S. Employer
incorporation or organization)                               Identification No.)


               607 East Adams Street, Springfield, Illinois 62739
              (Address of principal executive offices and Zip Code)


                         Registrant's telephone number,
                       including area code: (217) 523-3600


         Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the  Securities  Exchange  Act of
1934  during  the  preceding  12 months  (or for such  shorter  period  that the
registrant was required to file such reports),  and (2) has been subject to such
filing requirements for the past 90 days.


                              Yes   X.      No   .


Shares outstanding of each of registrant's classes of common stock as of October
  31, 1999:  Common  Stock,  no par value,  held by Ameren  Corporation  (parent
  company of Registrant) - 25,452,373


<PAGE>


                     Central Illinois Public Service Company

                                      Index

                                                                    Page No.

Part I       Financial Information (Unaudited)

             Management's Discussion and Analysis                       2

             Quantitative and Qualitative Disclosure
             About Market Risk                                          7

             Balance Sheet
             - September 30, 1999 and December 31, 1998                 9

             Statement of Income
             - Three months, Nine months, and 12 months ended
                September 30, 1999 and 1998                            10

             Statement of Cash Flows
             - Nine months ended September 30, 1999 and 1998           11

             Notes to Financial Statements                             12


Part II      Other Information                                         15


<PAGE>


2
                    PART I. FINANCIAL INFORMATION (UNAUDITED)

MANAGEMENT'S  DISCUSSION  AND  ANALYSIS OF  FINANCIAL  CONDITION  AND RESULTS OF
OPERATIONS

OVERVIEW

Central  Illinois  Public Service  Company  (AmerenCIPS or the  Registrant) is a
subsidiary of Ameren  Corporation  (Ameren),  a holding company registered under
the Public Utility Holding Company Act of 1935 (PUHCA).  In December 1997, Union
Electric Company  (AmerenUE) and CIPSCO  Incorporated  (CIPSCO) combined to form
Ameren,  with  AmerenUE and CIPSCO's  subsidiaries,  the  Registrant  and CIPSCO
Investment  Company (CIC),  becoming  wholly-owned  subsidiaries  of Ameren (the
Merger).

The following  discussion and analysis  should be read in  conjunction  with the
Notes  to  Financial  Statements  beginning  on page  12,  and the  Management's
Discussion and Analysis of Financial Condition and Results of Operations (MD&A),
the Audited Financial Statements and the Notes to Financial Statements appearing
in the Registrant's 1998 Form 10-K.

RESULTS OF OPERATIONS

Earnings
Third  quarter  1999  earnings of $42 million  increased  $3 million from 1998's
third quarter  earnings.  Earnings for the nine months ended  September 30, 1999
increased $7 million from the year-ago  period to $75 million.  Earnings for the
12 months ended September 30, 1999 were $84 million, a $36 million increase from
the preceding  12-month period.  Excluding the extraordinary  charge recorded in
the fourth quarter of 1997 to write off the generation-related regulatory assets
and liabilities of the Registrant's  retail electric business,  earnings for the
12-month period ended September 30, 1998 were $72 million.

Earnings  fluctuated  due to many  conditions,  primarily:  weather  variations,
credits to electric  customers, electric  rate  reductions,  competitive  market
forces,  sales growth,  fluctuating  operating costs,  merger-related  expenses,
changes in interest expense,  changes in income and property taxes, a charge for
a targeted employee separation plan and an extraordinary charge, as noted above.

The  significant  items  affecting  revenues,  costs  and  earnings  during  the
three-month,  nine-month and 12-month  periods ended September 30, 1999 and 1998
are detailed below.

Electric Operations

Electric Operating Revenues      Variations for periods ended September 30, 1999
                                         from comparable prior-year periods
-------------------------------------------------------------------------------
(Millions of Dollars)               Three Months   Nine Months   Twelve Months
-------------------------------------------------------------------------------
Credit to customers                  $   (8)        $  (8)         $   (8)
Rate variations                          (2)           (8)            (13)
Effect of abnormal weather               (1)          (15)            (25)
Growth and other                          -             8               1
Interchange sales                        42            71              80
-------------------------------------------------------------------------------
                                     $   31         $  48          $   35
-------------------------------------------------------------------------------


Electric  revenues for the three months ended September 30, 1999,  increased $31
million  compared to the prior  three-  month  period  primarily  driven by an 8
percent increase in interchange  sales, due to strong  marketing  efforts.  This
increase was partially  offset by a  residential  rate decrease and an estimated
credit that the Registrant expects to pay its electric customers for the initial
period of a sharing mechanism  provided by deregulation  legislation (see Note 5
under Notes to Financial Statements for further information).

Electric  revenues for the nine months and 12 months ended  September  30, 1999,
increased $48 million and $35 million, respectively,  compared to the comparable
year-ago periods,  primarily due to increases in interchange sales of 33 percent
and 23 percent,  respectively,  due to strong marketing efforts. These increases
were partially offset by a decrease in native sales of 2 percent for each of the
nine-month and 12-month periods,  primarily due to milder weather.  In addition,
electric revenues were reduced as a result of a residential rate decrease and an
estimated credit

                                      -2-

<PAGE>

that the Registrant expects to pay its electric customers for the initial period
of a sharing mechanism provided by deregulation legislation (see Note 5 under to
Notes to Financial Statements for further information).

Fuel and Purchased Power
                                 Variations for periods ended September 30, 1999
                                         from comparable prior-year periods
--------------------------------------------------------------------------------
(Millions of Dollars)                  Three Months  Nine Months  Twelve Months
--------------------------------------------------------------------------------
Fuel:
  Variation in generation                $   -         $  8          $  (3)
  Price                                     (4)         (16)           (19)
  Generation efficiencies and other         (2)          (4)            (9)
Purchased power variation                   21           37             39
--------------------------------------------------------------------------------
                                        $   15         $ 25          $   8
--------------------------------------------------------------------------------


Fuel and  purchased  power costs for the three months ended  September  30, 1999
increased  $15  million  over the same period in the prior year  primarily  as a
result of an increase in purchased  power  resulting  from higher sales  volume,
partially  offset by lower fuel prices.  Fuel and purchased  power costs for the
nine months ended  September 30, 1999  increased $25 million over the comparable
prior year period primarily due to an increase in generation and purchased power
resulting from higher sales volume,  partially offset by lower fuel prices.  The
$8 million  increase in fuel and  purchased  power costs for the 12 months ended
September  30, 1999 versus the  prior-year  period was  primarily  the result of
increased  purchased power resulting from higher sales volume,  partially offset
by lower fuel prices.

Gas Operations
Gas revenues for the 12-month  period ended  September  30, 1999,  decreased $17
million compared to the same year-ago period primarily due to a decline in total
sales  resulting  primarily from milder winter weather,  partially  offset by an
Illinois gas rate increase effective February 1999.

Gas costs for the nine months ended  September  30,  1999,  decreased $3 million
compared to the year-ago period due to lower sales.  Gas costs for the 12 months
ended September 30, 1999,  decreased $22 million compared to the year-ago period
primarily due to lower sales and lower gas prices.

Other Operating Expenses
Other operating expense variations  reflected  recurring factors such as growth,
inflation, labor and benefit increases.

Other operations expenses increased $3 million and $7 million, respectively, for
the three months and nine months ended September 30, 1999,  compared to the same
year-ago periods primarily due to increased  injuries and damages expenses based
on claims experience,  and expenses associated with deregulation in Illinois and
the Year 2000 project, partially offset by the 1998 one-time pretax charge of $7
million for the targeted employee  separation plan and reduced workforce.  Other
operations  expenses increased $12 million for the 12 months ended September 30,
1999,  compared to the same  year-ago  period,  primarily  due to  increases  in
expenses  associated  with  deregulation  in Illinois and the Year 2000 project,
partially offset by the 1998 charge for the targeted separation plan.

Maintenance  expenses  for the three  months,  nine  months and 12 months  ended
September  30,  1999,  increased  $9  million,  $17  million  and  $11  million,
respectively,  from the comparable year-ago periods due to increased power plant
maintenance.

Other Taxes
Other taxes decreased  primarily due to a decrease in gross receipt taxes.  This
decrease  results  from the  restructuring  of the Illinois  public  utility tax
whereby gross receipt taxes are no longer recorded as electric revenue and gross
receipt tax expense.

Taxes
Income taxes for the three months, nine months and 12 months ended September 30,
1999, increased $2 million, $3 million and $8 million,  respectively,  primarily
due to higher pretax income.

                                      -3-

<PAGE>

Balance Sheet
The $17  increase in trade  accounts  receivable  and  unbilled  revenue was due
primarily to higher  revenues in August and September  1999 compared to November
and December 1998.

Changes in accounts and wages payable,  taxes accrued,  other accounts and notes
receivable,  and other  current  assets  resulted  from the  timing  of  various
payments  to taxing  authorities  and  suppliers.  The $91  million  increase in
intercompany  notes  payable is due to funds  borrowed from a utility money pool
(see Note 6 under Notes to Financial Statements for further information).

The $11 million  increase in other current  liabilities was primarily due to the
estimated credit to Illinois electric customers recorded in the third quarter of
1999 for the initial  period of a sharing  mechanism  provided  by  deregulation
legislation  (see  Note 5  under  Notes  to  Financial  Statements  for  further
information).  The  remaining  variance  is a result of the  timing  of  various
payments to suppliers.

LIQUIDITY AND CAPITAL RESOURCES

Cash provided by operating  activities  totaled $163 million for the nine months
ended September 30, 1999, compared to $98 million during the same 1998 period.

Cash flows used in investing  activities totaled $81 million and $46 million for
the nine months ended  September 30, 1999 and 1998,  respectively.  Construction
expenditures  for the nine months ended September 30, 1999, for constructing new
or improving existing facilities were $81 million.  Capital requirements for the
remainder of 1999 are expected to be principally for construction expenditures.

Cash flows used in financing  activities totaled $72 million for the nine months
ended  September 30, 1999,  compared to $67 million during the same 1998 period.
The  Registrant's  principal  financing  activities for the period  included the
redemption  of $107  million  of debt and the  payment of  dividends,  partially
offset by the issuance of intercompany notes payable.

The Registrant  plans to continue  utilizing  short-term  debt to support normal
operations and other temporary requirements. The Registrant is authorized by the
Securities  and  Exchange  Commission  under PUHCA to have up to $250 million of
short-term  unsecured debt instruments  outstanding at any one time.  Short-term
borrowings  consist of bank loans  (maturities  generally on an overnight basis)
and commercial paper  (maturities  generally within 10 to 45 days). At September
30, 1999,  the Registrant  had committed  bank lines of credit  aggregating  $30
million  (all of which  was  unused  and  available  at such  date)  which  make
available  interim  financing at various rates of interest  based on LIBOR,  the
bank  certificate  of  deposit  rate or other  options.  The lines of credit are
renewable  annually at various dates throughout the year. At September 30, 1999,
the Registrant had no outstanding  short-term  borrowings other than $91 million
of intercompany notes payable.

Also,  Ameren has a bank credit  agreement due 2002, which permits the borrowing
of up to $200 million on a short-term  basis. This credit agreement is available
to Ameren and its  subsidiaries,  including the Registrant.  As of September 30,
1999, $132 million was available for the Registrant's use.

The Registrant,  in the ordinary course of business,  explores  opportunities to
reduce its cost in order to remain  competitive in the marketplace.  Areas where
the Company focuses its review include,  but are not limited to, labor costs and
fuel  supply  costs.   In  the  labor  area,  the  Registrant  is  currently  in
negotiations with many of the Registrant's major collective  bargaining units in
an effort to manage its labor costs and practices effectively in the future. The
Registrant  also explores  alternatives  to  effectively  manage the size of its
workforce. These alternatives include utilizing hiring freezes,  outsourcing and
offering employee separation  packages.  In the fuel supply area, the Registrant
explores  alternatives  to  effectively  manage its overall  fuel  costs.  These
alternatives  include  diversifying  fuel  sources  for use at the  Registrant's
fossil plants (e.g.  utilizing  low sulfur versus high sulfur coal),  as well as
restructuring or terminating existing contracts with suppliers.

Certain of these reduction  alternatives could result in additional  investments
being made at the Registrant's  power plants in order to utilize different types
of coal, or could require nonrecurring  payments of employee separation benefits
or  nonrecurring  payments to restructure or terminate an existing fuel contract
with a supplier.  Management  is unable to predict  which (if any),  and to what
extent,  these  alternatives  to  reduce  its  overall  cost  structure  will be
executed,  as well as determine the impact of these actions on the  Registrant's
future financial position, results of operations or cash flows.

                                      -4-

<PAGE>

RATE MATTERS

In March 1999, the Registrant  filed delivery  service tariffs with the Illinois
Commerce  Commission  (ICC) to  comply  with the  requirements  of the  Electric
Service Customer Choice and Rate Relief Law of 1997. These tariffs would be used
by  electric  customers  who choose to purchase  their  power from an  alternate
supplier.  On August 25, 1999,  the ICC issued an order  approving  the delivery
services  tariffs,  with an  allowed  rate of return on  equity of  10.45%.  The
Registrant  and  AmerenUE  filed a joint  petition  for  rehearing of that order
requesting the ICC to alter its  conclusions  on a number of issues.  On October
13,  1999,  the ICC  granted a  rehearing  on certain  issues.  An order on this
reopened proceeding is expected in early 2000.

In August 1999,  Ameren filed a  transmission  system rate case with the Federal
Energy  Regulatory  Commission  (FERC).  This filing was  primarily  designed to
implement rates, terms and conditions for transmission  service for those retail
customers in Illinois  which choose other  suppliers.  On October 14, 1999,  the
FERC issued an order suspending the proposed rates until March 25, 2000.  Ameren
filed in response an emergency  request for rehearing  which  requested that the
portion of the filing which  related to retail access in Illinois be placed into
effect as of October 1, 1999 to coincide with the start of retail competition in
Illinois. An order from the FERC to this request is expected shortly. An initial
decision as to Ameren's overall filing is expected in early 2001.

See Note 5 under Notes to Financial  Statements  for further  discussion of Rate
Matters.

ELECTRIC INDUSTRY RESTRUCTURING

In December 1997, the Governor of Illinois signed the Electric  Service Customer
Choice and Rate  Relief Law of 1997 (the Law)  providing  for  electric  utility
restructuring  in Illinois.  This  legislation  introduces  competition into the
supply of electric energy in Illinois.

One of the major  provisions of the Law includes the phasing-in  through 2002 of
retail direct access,  which allows customers to choose their electric supplier.
The  phase-in  of retail  direct  access  began on October  1, 1999,  with large
commercial and industrial  customers  principally  comprising the initial group.
The  customers  in  this  group  represent   approximately  24  percent  of  the
Registrant's total sales.  Retail direct access will be offered to the remaining
commercial  and  industrial  customers on December 31, 2000,  and to residential
customers on May 1, 2002.

In conjunction with another provision of the Law, in July 1999, AmerenCIPS filed
a  notice  with the ICC  that it  intends  to  transfer  AmerenCIPS'  generating
facilities  (all in Illinois) to a new  unregulated  subsidiary  of Ameren.  The
formation  of  the  new  generating  subsidiary,  as  well  as the  transfer  of
AmerenCIPS' generating assets and liabilities (at historical net book value) and
certain  power  sales  contracts,  will be  subject to  regulatory  proceedings.
Regulatory  approval  was  received  from the ICC on  October  13,  1999.  Other
regulatory approvals are required from the Federal Energy Regulatory  Commission
and the  Missouri  Public  Service  Commission.  In addition  to the  AmerenCIPS
facilities,  the generating  subsidiary  will include  substantially  all of the
combustion  turbine  generators  which Ameren has committed to acquire.  The new
subsidiary  is  expected  to be  operational  sometime  in 2000,  subject to the
outcome of these regulatory proceedings.

Once the  transfer is  completed,  a power  supply  agreement  would be in place
between the new generating company and a nonregulated  marketing  subsidiary for
all generation.  The marketing  subsidiary  would have a power supply  agreement
with  AmerenCIPS  to supply  them  sufficient  generation  to meet  native  load
requirements  over the term of the agreement.  Power will continue to be jointly
dispatched between AmerenUE, AmerenCIPS and the new generating subsidiary.

The proposed  transfer of generating assets and liabilities had no effect on the
Registrant's financial statements as of September 30, 1999.

YEAR 2000 ISSUE

The Year  2000  Issue  relates  to how  dates are  stored  and used in  computer
systems,  applications, and embedded systems. As the century date change occurs,
certain date-sensitive systems need to be able to recognize the year as 2000 and
not as 1900. This inability to recognize and properly treat the year as 2000 may
cause these systems to process  critical  financial and operational  information
incorrectly.   The  Registrant's  primary  concern  is  the  potential  for  any
interruption in providing electric and gas service to customers,  as well as the
potential inability to process critical financial and operational information on
a timely basis, including billing its customers, if appropriate steps

                                      -5-

<PAGE>

are not taken to address this issue.  Management  has developed a Year 2000 plan
(Plan) covering Ameren,  including  AmerenCIPS,  and Ameren's Board of Directors
has been briefed about the Year 2000 Issue and how it may affect the Registrant.

Ameren's Plan to resolve the Year 2000 Issue involves three phases:  assessment,
planning,  and implementation/  testing.  Implementation of the Plan is directly
supervised  by each  area's  responsible  Vice  President.  A Year 2000  Project
Director  coordinates the  implementation of the Plan among functional teams who
are  addressing  issues  specific  to a  particular  area,  such as nuclear  and
non-nuclear generation facilities,  energy management systems, gas distribution,
etc. Ameren has also engaged certain outside consultants,  technicians and other
external resources to aid in formulating and implementing the Plan.

Ameren  has  completed  its   assessment   phase,   which   included   analyzing
date-sensitive  electronic hardware,  software applications and embedded systems
and has developed a compliance plan to address issues that were identified. Many
of the major  corporate  computer  systems  at  Ameren  are  relatively  new and
therefore are either Year 2000  compliant or only require  minor  modifications.
Also,   several  of  the  operating   hardware  and  embedded   systems   (i.e.,
microprocessor  chips) use analog  rather than digital  technology  and thus are
unaffected  by the  two-digit  date issue.  In  addition,  Ameren has  contacted
hundreds of vendors and suppliers to verify compliance.

Ameren has also completed its planning  phase.  Items that have been  identified
for remediation have been prioritized into groups based on their significance to
Ameren's operations.

The implementation/testing  phase for all mission critical systems was completed
by  September  30,  1999.  The   implementation/testing   phase  for  all  other
components/applications is approximately 98 percent complete as of September 30,
1999.

With  respect  to  third  parties,   for  areas  that  interface  directly  with
significant  vendors,  Ameren has inventoried vendors and major suppliers and is
currently  assessing  their Year 2000 readiness  through  surveys,  websites and
personal contact. Ameren plans to follow up with major suppliers and vendors and
verify Year 2000  compliance,  where  appropriate.  Ameren has also  queried its
health insurance  providers.  To date,  Ameren is not aware of any problems that
would  materially  impact its  financial  condition,  results of  operations  or
liquidity.  However, neither Ameren nor the Registrant has the means of ensuring
that these parties will be Year 2000  compliant.  The inability of those parties
to complete their Year 2000 resolution  process could  materially  impact Ameren
and the Registrant.

Ameren has also  addressed  the impact of electric  power grid problems that may
occur  outside  of its own  electric  system.  Ameren  has  conducted  Year 2000
electric  power grid impact  planning  through  the  system's  various  electric
interconnection  affiliations and is working with the  Mid-American  Interchange
Network (MAIN) and the North  American  Electric  Reliability  Council (NERC) to
plan  Year  2000  operational  preparedness  and  restoration  scenarios.  As of
September  30,  1999,  Ameren  has  completed  its  assessment,   planning,  and
implementation/testing  phases for mission critical items as identified by NERC.
As a result, Ameren has been added to the "Ready" list being compiled by NERC as
it assesses  readiness of the regional and national  electric grid.  Through the
Electric Power  Research  Institute  (EPRI),  an  industry-wide  effort has been
established  to deal with Year  2000  problems  affecting  digital  systems  and
equipment  used by the nation's  electric  power  companies.  Under this effort,
participating  utilities are working together to assess specific vendors' system
problems and test plans.  The assessment is being shared with EPRI  participants
to facilitate Year 2000 problem solving.

In addressing  the Year 2000 Issue,  Ameren will incur  internal  labor costs as
well as external  consulting  and other expenses to prepare for the new century.
Ameren estimates that its external costs (consulting fees and related costs) for
addressing the Year 2000 Issue will range from $10 million to $15 million. As of
September 30, 1999, Ameren had expended approximately $8 million. Ameren's plans
to complete Year 2000  modifications  are based on management's  best estimates,
which are derived utilizing numerous  assumptions of future events including the
continued availability of certain resources,  and other factors.  However, there
can be no guarantee  that these  estimates  will be achieved and actual  results
could differ materially from those plans. Specific factors that might cause such
material  differences include, but are not limited to, the availability and cost
of  personnel  trained in this area,  the  ability  to locate  and  correct  all
relevant computer codes, and similar uncertainties.

Ameren  believes  that,  with  appropriate  modifications  to existing  computer
systems/components,  updates by vendors and trading partners,  and conversion to
new  software and  hardware in the  ordinary  course of business,  the Year 2000
Issue  will  not  pose  significant  operational  problems  for the  Registrant.
However,  if such conversions are not completed in a proper and timely manner by
all  affected  parties,  the Year 2000 Issue could  result in  material  adverse
operational and financial  consequences  to the Registrant,  and there can be no
assurance that Ameren's efforts, or

                                      -6-

<PAGE>

those of vendors and trading partners,  interconnection affiliates, NERC or EPRI
to address the Year 2000 Issue will be  successful.  Ameren is in the process of
developing  contingency  plans to address  potential  risks,  including risks of
vendor/trading  partners  noncompliance,  as well as noncompliance of any of the
Registrant's material operating systems. The first operational  contingency plan
addressing  power grid issues was  completed  during the first  quarter of 1999.
Contingency  plans related to the business areas were completed in July 1999. At
this time, the Registrant is unable to predict the ultimate  impact,  if any, of
the  Year  2000  Issue  on the  Registrant's  financial  condition,  results  of
operations or liquidity; however, the impact could be material.

ACCOUNTING MATTERS

In June 1998, the Financial  Accounting  Standards Board (FASB) issued Statement
of  Financial  Accounting  Standards  (SFAS)  133,  "Accounting  for  Derivative
Instruments  and  Hedging  Activities."  SFAS  133  establishes  accounting  and
reporting  standards for derivative  instruments and for hedging  activities and
requires  recognition  of all  derivatives on the balance sheet measured at fair
value.  In June 1999,  the FASB  issued  SFAS 137,  "Accounting  for  Derivative
Instruments  and  Hedging  Activities--Deferral  of the  Effective  Date of FASB
Statement No. 133," which  delayed the effective  date of SFAS 133 to all fiscal
quarters of all fiscal years beginning after June 15, 2000. Earlier  application
is still  encouraged.  At this time,  the  Registrant is unable to determine the
impact of SFAS 133 on its  financial  position  or  results of  operations  upon
adoption;  however,  SFAS 133 could increase the volatility of the  Registrant's
future earnings.

QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

Market risk  represents the risk of changes in value of a financial  instrument,
derivative  or  non-derivative,  caused by  fluctuations  in interest  rates and
equity prices. The following discussion of Ameren's, including AmerenCIPS', risk
management activities includes  "forward-looking"  statements that involve risks
and  uncertainties.  Actual results could differ materially from those projected
in the "forward-looking"  statements.  Ameren handles market risks in accordance
with established  policies,  which may include entering into various  derivative
transactions. In the normal course of business, Ameren also faces risks that are
either non-financial or non-quantifiable.  Such risks principally include credit
risk and legal risk and are not represented in the following analysis.

Interest Rate Risk
The  Registrant  is exposed to market risk  through  changes in  interest  rates
through its  issuance  of both  long-term  and  short-term  variable-rate  debt,
fixed-rate  debt,  commercial  paper and auction  market  preferred  stock.  The
Registrant manages its interest rate exposure by controlling the amount of these
instruments it holds within its total capitalization portfolio and by monitoring
the effects of market changes in interest rates.

If  interest  rates  increase  1  percent  in  2000 as  compared  to  1999,  the
Registrant's  interest  expense would  increase and net income would decrease by
approximately $1 million.  This amount has been determined using the assumptions
that the  Registrant's  outstanding  variable  rate debt,  commercial  paper and
auction  market  preferred  stock as of  September  30,  1999,  continued  to be
outstanding  throughout  2000,  and that the  average  interest  rates for these
instruments  increased  1 percent  over 1999.  The model does not  consider  the
effects of the reduced  level of overall  economic  activity that would exist in
such an  environment.  In the event of a significant  change in interest  rates,
management  would likely take  actions to further  mitigate its exposure to this
market risk. However,  due to the uncertainty of the specific actions that would
be taken and their possible effects,  the sensitivity analysis assumes no change
in the Registrant's financial structure.

Commodity Price Risk
The  Registrant  is exposed to changes in market prices for natural gas and fuel
and  purchased  power.  With regard to its natural  gas  utility  business,  the
Registrant's  exposure to changing  market prices is in large part  mitigated by
the fact that the  Registrant  has a Purchased  Gas  Adjustment  Clause (PGA) in
place.  The PGA allows the  Registrant to pass on to its customers its prudently
incurred costs of natural gas.

Since  the  Registrant  does not  have a  provision  similar  to the PGA for its
electric operations, the Registrant has entered into several long-term contracts
with various  suppliers to purchase  coal to manage its exposure to fuel prices.
With regard to the Registrant's  exposure to commodity risk for purchased power,
Ameren  has  established  a  subsidiary,   AmerenEnergy,   Inc.,  whose  primary
responsibility  includes  managing  market  risks  associated  with the changing
market prices for purchased power for the Registrant.

AmerenEnergy  utilizes  several  techniques  to  mitigate  its  market  risk for
purchased  power,  including  utilizing  derivative  financial  instruments.   A
derivative  is a contract  whose value is dependent on or derived from the value
of

                                      -7-

<PAGE>

some underlying asset. The derivative financial instruments that AmerenEnergy is
allowed to utilize (which include forward  contracts and futures  contracts) are
dictated by a risk management policy,  which has been reviewed with the Auditing
Committee of Ameren's Board of Directors.  Compliance  with the risk  management
policy is the responsibility of a risk management steering committee, consisting
of Ameren officers and an independent risk management officer at AmerenEnergy.

As of September  30, 1999,  the fair value of derivative  financial  instruments
exposed to commodity price risk was immaterial.

SAFE HARBOR STATEMENT

Statements made in this Form 10-Q 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,
financial  performance  and the Year 2000 Issue.  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 Annual  Report on Form 10-K for the fiscal year ended  December 31, 1998,
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;  the impact on the  Registrant  of current  regulations  related to the
phasing-in of the  opportunity for some customers to choose  alternative  energy
suppliers in Illinois;  the effects of increased  competition  in the future due
to, among other  things,  deregulation  of certain  aspects of the  Registrant's
business at both the state and Federal levels; future market prices for fuel and
purchased power,  electricity,  and natural gas,  including the use of financial
instruments; average rates for electricity in the Midwest; business and economic
conditions;  interest rates;  weather conditions;  fuel prices and availability;
generation plant performance; the impact of current environmental regulations on
utilities  and  the  expectation  that  more  stringent   requirements  will  be
introduced over time, which could potentially have a negative  financial effect;
monetary and fiscal  policies;  future wages and employee  benefits  costs;  and
legal and administrative proceedings.

                                      -8-

<PAGE>





                     CENTRAL ILLINOIS PUBLIC SERVICE COMPANY
                                  BALANCE SHEET
                                    UNAUDITED
                      (Thousands of Dollars, Except Shares)

<TABLE>
<CAPTION>

                                                             September 30,   December 31,
ASSETS                                                            1999         1998
------                                                       -------------   ------------
<S>                                                          <C>           <C>
Property and plant, at original cost:
   Electric                                                     $2,400,383   $2,381,682
   Gas                                                             265,459      259,656
                                                                ----------   ----------
                                                                 2,665,842    2,641,338
   Less accumulated depreciation and amortization                1,246,472    1,192,108
                                                                ----------   ----------
                                                                 1,419,370    1,449,230
Construction work in progress                                       61,370       16,220
                                                                ----------   ----------
         Total property and plant, net                           1,480,740    1,465,450
                                                                ----------   ----------

Other assets                                                        29,526       31,904

Current assets:
   Cash and cash equivalents                                        21,351       10,180
   Accounts receivable - trade (less allowance for doubtful
         accounts of $2,593 and $1,714, respectively)               52,191       44,494
   Unbilled revenue                                                 62,397       53,120
   Other accounts and notes receivable                              41,797       16,486

   Materials and supplies, at average cost -
      Fossil fuel                                                   46,290       50,791
      Other                                                         35,181       36,047
   Other                                                            10,602        8,214
                                                                ----------   ----------
         Total current assets                                      269,809      219,332
                                                                ----------   ----------
Regulatory assets:
   Deferred income taxes                                            22,339       24,797
   Other                                                            19,214       22,914
                                                                ----------   ----------
         Total regulatory assets                                    41,553       47,711
                                                                ----------   ----------
Total Assets                                                    $1,821,628   $1,764,397
                                                                ==========   ==========

CAPITAL AND LIABILITIES
Capitalization:
   Common stock, no par value, authorized 45,000,000 shares -
     outstanding 25,452,373 shares                              $  120,033   $  120,033
   Retained earnings                                               477,525      455,337
                                                                ----------   ----------
         Total common stockholders' equity                         597,558      575,370
   Preferred stock not subject to mandatory redemption              80,000       80,000
   Long-term debt                                                  493,586      528,446
                                                                ----------   ----------
        Total capitalization                                     1,171,144    1,183,816
                                                                ----------   ----------
Current liabilities:
   Current maturity of long-term debt                               35,000       60,000
   Short-term debt                                                    --         46,700
   Intercompany notes payable                                       91,200         --
   Accounts and wages payable                                       88,743       61,609
   Accumulated deferred income taxes                                22,312       21,386
   Taxes accrued                                                    26,799       13,201
   Other                                                            45,200       34,454
                                                                ----------   ----------
         Total current liabilities                                 309,254      237,350
                                                                ----------   ----------
Accumulated deferred income taxes                                  222,174      234,119
Accumulated deferred investment tax credits                         32,794       34,657
Regulatory liability                                                35,409       39,621
Other deferred credits and liabilities                              50,853       34,834
                                                                ----------   ----------
Total Capital and Liabilities                                   $1,821,628   $1,764,397
                                                                ==========   ==========

</TABLE>

See Notes to Financial Statements.

                                      -9-

<PAGE>


                     CENTRAL ILLINOIS PUBLIC SERVICE COMPANY
                               STATEMENT OF INCOME
                                    UNAUDITED
                             (Thousands of Dollars)


<TABLE>
<CAPTION>


                                              Three Months Ended         Nine Months Ended        Twelve Months Ended
                                                  September 30,            September 30,              September 30,
                                             ----------------------    ----------------------    ----------------------
                                                1999         1998         1999         1998         1999         1998
                                                ----         ----         ----         ----         ----         ----

<S>                                       <C>          <C>          <C>          <C>          <C>          <C>
 OPERATING REVENUES:
    Electric                                $ 261,453    $ 230,422    $ 617,029    $ 568,866    $ 770,081    $ 734,692
    Gas                                        17,874       16,253       91,999       92,153      125,352      142,534
                                            ---------    ---------    ---------    ---------    ---------    ---------
 Total operating revenues                     279,327      246,675      709,028      661,019      895,433      877,226

 OPERATING EXPENSES:
    Operations
       Fuel and purchased power                82,072       66,965      208,304      183,143      255,246      246,763
       Gas                                      7,324        6,517       50,326       53,372       66,304       87,657
       Other                                   54,944       51,941      140,923      134,092      186,308      174,071
                                            ---------    ---------    ---------    ---------    ---------    ---------
                                              144,340      125,423      399,553      370,607      507,858      508,491
    Maintenance                                24,741       16,232       67,309       50,153       88,698       77,749
    Depreciation and amortization              20,141       17,357       60,330       54,791       79,862       75,233
    Income taxes                               26,204       24,484       43,380       40,113       49,036       40,970
    Other taxes                                10,056       12,556       29,921       44,571       42,184       58,668
                                            ---------    ---------    ---------    ---------    ---------    ---------
       Total operating expenses               225,482      196,052      600,493      560,235      767,638      761,111

 OPERATING INCOME                              53,845       50,623      108,535      100,784      127,795      116,115

 OTHER INCOME AND DEDUCTIONS:
    Allowance for equity funds used
       during construction                         (9)          (4)          (8)          45          (37)         447
    Miscellaneous, net                            533         (834)       1,614         (545)       1,204       (3,713)
                                            ---------    ---------    ---------    ---------    ---------    ---------
        Total other income and deductions         524         (838)       1,606         (500)       1,167       (3,266)

 INCOME BEFORE
    INTEREST CHARGES                           54,369       49,785      110,141      100,284      128,962      112,849

 INTEREST CHARGES:
    Interest                                   10,748       10,423       31,906       30,126       41,819       38,472
    Allowance for borrowed funds
       used during construction                   338         (310)         (43)        (981)        (143)      (1,283)
                                            ---------    ---------    ---------    ---------    ---------    ---------
    Net interest charges                       11,086       10,113       31,863       29,145       41,676       37,189

 INCOME BEFORE
    EXTRAORDINARY CHARGE                       43,283       39,672       78,278       71,139       87,286       75,660
                                            ---------    ---------    ---------    ---------    ---------    ---------

 EXTRAORDINARY CHARGE
   (NET OF INCOME TAXES)                         --           --           --           --           --        (24,853)
                                            ---------    ---------    ---------    ---------    ---------    ---------
NET INCOME                                     43,283       39,672       78,278       71,139       87,286       50,807

PREFERRED STOCK DIVIDENDS                         957          936        2,842        2,801        3,786        3,734
                                            ---------    ---------    ---------    ---------    ---------    ---------

NET INCOME AFTER PREFERRED
   STOCK DIVIDENDS                          $  42,326    $  38,736    $  75,436    $  68,338    $  83,500    $  47,073
                                            =========    =========    =========    =========    =========    =========

</TABLE>


See Notes to Financial Statements.

                                      -10-

<PAGE>

                     CENTRAL ILLINOIS PUBLIC SERVICE COMPANY
                             STATEMENT OF CASH FLOWS
                                    UNAUDITED
                             (Thousands of Dollars)

<TABLE>
<CAPTION>


                                                            Nine Months Ended
                                                             September 30,
                                                           1999         1998
<S>                                                  <C>          <C>
Cash Flows From Operating:
   Net income                                          $  78,278    $  71,139
   Adjustments to reconcile net income to net cash
      provided by operating activities:
        Depreciation and amortization                     60,330       54,791
        Allowance for funds used during construction         (35)      (1,026)
        Deferred income taxes, net                       (12,774)      (9,760)
        Deferred investment tax credits, net              (1,863)      (4,573)
        Changes in assets and liabilities:
           Receivables, net                              (42,285)     (36,417)
           Materials and supplies                          5,367       (6,942)
           Accounts and wages payable                     27,134      (13,590)
           Taxes accrued                                  13,598        9,186
           Other, net                                     35,741       35,163
                                                       ---------    ---------
Net cash provided by operating activities                163,491       97,971

Cash Flows From Investing:
   Construction expenditures                             (80,601)     (46,911)
   Allowance for funds used during construction               35        1,026
                                                       ---------    ---------
Net cash used in investing activities                    (80,566)     (45,885)

Cash Flows From Financing:
   Dividends on common stock                             (53,297)     (53,297)
   Dividends on preferred stock                           (2,957)      (3,035)
   Redemptions -
      Short-term debt                                    (46,700)     (11,166)
      Long-term debt                                     (60,000)     (10,000)
   Issuances -
      Long-term debt                                        --         10,000
      Intercompany notes payable                          91,200         --
                                                       ---------    ---------
Net cash used in financing activities                    (71,754)     (67,498)
                                                       ---------    ---------

Net increase (decrease) in cash and cash equivalents      11,171      (15,412)
Cash and cash equivalents at beginning of year            10,180       28,140
                                                       ---------    ---------
Cash and cash equivalents at end of period             $  21,351    $  12,728
                                                       =========    =========

Cash paid during the periods:
   Interest (net of amount capitalized)                $  28,990    $  29,650
   Income taxes, net                                   $  39,983    $  37,940

</TABLE>

See Notes to Financial Statements.

                                      -11-

<PAGE>



CENTRAL ILLINOIS PUBLIC SERVICE COMPANY
NOTES TO FINANCIAL STATEMENTS (UNAUDITED)
September 30, 1999


Note 1 - Central Illinois Public Service Company  (AmerenCIPS or the Registrant)
is a wholly-owned subsidiary of Ameren Corporation (Ameren), which is the parent
company of two utility  operating  companies,  the Registrant and Union Electric
Company  (AmerenUE).  Ameren is a registered  holding  company  under the Public
Utility  Holding  Company Act of 1935 (PUHCA)  formed in December  1997 upon the
merger of CIPSCO Incorporated (the Registrant's former parent) and AmerenUE (the
Merger).  Both  Ameren  and  its  subsidiaries  are  subject  to the  regulatory
provisions of the PUHCA. The operating  companies are engaged principally in the
generation,  transmission,  distribution  and sale of  electric  energy  and the
purchase, distribution,  transportation and sale of natural gas in the states of
Illinois and Missouri. Contracts among the companies--dealing with jointly-owned
generating facilities,  interconnecting  transmission lines, and the exchange of
electric power--are regulated by the Federal Energy Regulatory Commission (FERC)
or the Securities and Exchange Commission (SEC). Administrative support services
are provided to the Registrant by a separate Ameren subsidiary,  Ameren Services
Company.  The Registrant  serves 400,000 electric and 175,000 gas customers in a
20,000 square-mile region of central and southern Illinois.

The Registrant also has a 20% interest in Electric Energy,  Inc. (EEI), which is
accounted  for under the equity method of  accounting.  EEI owns and operates an
electric generating and transmission facility in Illinois that supplies electric
power primarily to a uranium enrichment plant located in Paducah, Kentucky.

Note 2 - Financial  statement note  disclosures,  normally included in financial
statements prepared in conformity with generally accepted accounting principles,
have been omitted in this Form 10-Q pursuant to the Rules and Regulations of the
Securities and Exchange  Commission.  However, in the opinion of the Registrant,
the disclosures contained in this Form 10-Q are adequate to make the information
presented not misleading. See Notes to Financial Statements included in the 1998
Form 10-K for information relevant to the financial statements contained in this
Form 10-Q,  including  information as to the significant  accounting policies of
the Registrant.

Note 3 - In the  opinion of the  Registrant,  the interim  financial  statements
filed as part of this Form 10-Q  reflect  all  adjustments,  consisting  only of
normal recurring adjustments,  necessary for a fair statement of the results for
the periods presented.  The Registrant's  financial  statements were prepared to
permit the information  required in the Financial Data Schedule  (FDS),  Exhibit
27,  to be  directly  extracted  from  the  filed  statements.  The FDS  amounts
correspond  to or are  calculable  from the amounts  reported  in the  financial
statements or notes thereto.

Note 4 - Due to the  effect of  weather  on sales and  other  factors  which are
characteristic of public utility  operations,  financial results for the periods
ended September 30, 1999 and 1998, are not necessarily  indicative of trends for
any three-month, nine-month or 12-month period.

Note 5 -In conjunction with the Electric Service Customer Choice and Rate Relief
Law of 1997 (the Law), a 5 percent  residential  electric  rate decrease for the
Registrant's electric customers was effective August 1, 1998. This rate decrease
is  expected to  decrease  electric  revenues  $11  million  annually,  based on
estimated levels of sales and assuming normal weather conditions. The Registrant
may be subject to additional 5 percent  residential  electric rate  decreases in
each of 2000 and 2002,  to the  extent  its rates  exceed  the  Midwest  utility
average at that time.  The  Registrant's  rates are currently  below the Midwest
utility average.

The Law also contains a provision requiring one-half of excess earnings from the
Illinois  jurisdiction  for the years 1998  through  2004 to be  refunded to the
Registrant's  customers.  Excess  earnings  are  defined  as the  portion of the
two-year average annual rate of return on common equity in excess of 1.5 percent
of the two-year average of an Index, as defined in the Law. The Index is defined
as the sum of the  average  for the  twelve  months  ended  September  30 of the
average  monthly  yields of the  30-year U. S.  Treasury  bonds plus  prescribed
percentages  ranging from 4 percent to 5 percent.  In July 1999,  Senate Bill 24
was passed which increased the prescribed  percentages to 7 percent beginning in
2000.  Filings must be made with the Illinois  Commerce  Commission on or before
March  31 of each  year  2000  through  2005.  As of  September  30,  1999,  the
Registrant  recorded an estimated $8 million credit that it expects to return to
its customers under the Law for the two year period ended December 31, 1999.

                                      -12-

<PAGE>

Note 6 - The Registrant has  transactions  in the normal course of business with
other Ameren  subsidiaries.  These transactions are primarily comprised of power
purchases and sales and services received or rendered.  Intercompany receivables
included in other accounts and notes receivable were  approximately  $32 million
and $2 million,  respectively,  as of September  30, 1999 and December 31, 1998.
Intercompany   payables   included  in  accounts  and  wages   payable   totaled
approximately  $33 and $12 million,  respectively,  as of September 30, 1999 and
December 31, 1998.

In March 1999, the Registrant,  along with Ameren Services Company and AmerenUE,
entered  into a utility  money pool  agreement  to  coordinate  and  provide for
certain short-term cash and working capital  requirements.  Borrowings under the
agreement  are limited to $500 million and are due on demand or within one year.
Interest is calculated at varying rates depending on the composition of internal
and external funds in the money pool. The money pool is  administered  by Ameren
Services  Company.  The Registrant  recorded an intercompany note payable of $91
million, representing funds borrowed from the utility money pool as of September
30, 1999.

Note 7 - Statement of Position (SOP) 98-1, "Accounting for the Costs of Computer
Software  Developed or Obtained for Internal Use" became effective on January 1,
1999.  SOP 98-1  provides  guidance  on  accounting  for the  costs of  computer
software  developed or obtained for internal use. Under SOP 98-1, certain costs,
may be capitalized and amortized over some future period.  SOP 98-1 did not have
a  material  impact  on  the  Registrant's  financial  position  or  results  of
operations upon adoption.

The  Emerging  Issues Task Force of the  Financial  Accounting  Standards  Board
(EITF)  Issue  98-10,   "Accounting  for  Energy  Trading  and  Risk  Management
Activities" became effective on January 1, 1999. EITF 98-10 provides guidance on
the  accounting  for energy  contracts  entered into for the purchase or sale of
electricity,  natural  gas,  capacity  and  transportation.  The EITF  reached a
consensus in EITF 98-10 that sales and purchase  activities being performed need
to be classified as either  trading or  non-trading.  Furthermore,  transactions
that are determined to be trading  activities would be recognized on the balance
sheet measured at fair value,  with gains and losses included in earnings.  EITF
98-10  includes  factors  or  indicators  to  consider  when  determining  if  a
transaction is a trading or non-trading activity. Currently, AmerenEnergy, Inc.,
an energy marketing subsidiary of Ameren, enters into contracts for the sale and
purchase of energy on behalf of AmerenCIPS.  These  transactions  are considered
non-trading  activities  and are  accounted  for using the accrual or settlement
method, which represents industry practice.  Should any of AmerenEnergy's future
activities be considered  material  trading  activities  based on the indicators
provided in EITF 98-10, a change in accounting practice would be required.  EITF
98-10 did not have a material impact on the Registrant's  financial  position or
results of operations upon adoption.

                                      -13-


<PAGE>


<TABLE>
<CAPTION>

Note 8 -  Segment  information  for the  three  month,  nine  month and 12 month
periods ended September 30, 1999 and 1998 is as follows:

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

(in thousands)                                Electric          Gas            Total
----------------------------------------------------------------------------------------

Three months ended September 30, 1999:

<S>                                       <C>               <C>             <C>
Revenues                                    $261,453          $17,874         $279,327
Operating Income                              53,763               82           53,845
----------------------------------------------------------------------------------------

Three months ended September 30, 1998:

Revenues                                    $230,422          $16,253         $246,675
Operating Income (Loss)                       50,880             (257)          50,623
----------------------------------------------------------------------------------------

Nine months ended September 30, 1999:

Revenues                                    $617,029          $91,999         $709,028
Operating Income                             102,182            6,353          108,535
----------------------------------------------------------------------------------------

Nine months ended September 30, 1998:

Revenues                                    $568,866          $92,153         $661,019
Operating Income                              97,806            2,978          100,784
----------------------------------------------------------------------------------------

12 months ended September 30, 1999:

Revenues                                    $770,081         $125,352         $895,433
Operating Income                             119,358            8,437          127,795
----------------------------------------------------------------------------------------

12 months ended September 30, 1998:

Revenues                                    $734,692         $142,534         $877,226
Operating Income                             109,773            6,342          116,115
----------------------------------------------------------------------------------------

</TABLE>


Note 9 - Certain  reclassifications were made to prior-year financial statements
to conform to current-period presentation.

                                      -14-

<PAGE>



                           PART II. OTHER INFORMATION


ITEM 5.  OTHER INFORMATION

         On August 26, 1999,  the Board of Directors of the  Registrant  amended
its By-Laws to change the date for holding its annual meeting of stockholders to
the fourth  Tuesday of April in each year to  coincide  with the annual  meeting
date of its parent, Ameren Corporation.

         In addition,  the By-Laws were  amended to include a  requirement  that
stockholders who intend to submit a proposal in person at an annual meeting,  or
who intend to nominate a director at a meeting,  must  provide  advance  written
notice along with other prescribed information.  In general, such notice must be
received by the Secretary of the  Registrant  not later than 60 nor earlier than
90 days prior to the first  anniversary of the preceding  year's annual meeting.
Consequently,  for the Registrant's  annual meeting of stockholders for the year
2000,  stockholders  who intend to submit a  stockholder  proposal  or  director
nomination  in person at the meeting  must provide  written  notice by not later
than February 22, 2000 or earlier than January 23, 2000.

         Any stockholder  proposal  intended for inclusion in the proxy material
for the Registrant's 2000 annual meeting of stockholders must be received by the
Registrant by December 1, 1999.


ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K

         (a)   Exhibits.

               Exhibit 3(ii)-By-Laws of Central Illinois Public Service Company,
               as amended as of August 26, 1999.

               Exhibit 12 -  Computation  of Ratio of Earnings to Fixed  Charges
               and  Preferred  Stock  Dividend  Requirements,  12  Months  Ended
               September 30, 1999.

               Exhibit 27 - Financial Data Schedule.

         (b)   Reports on Form 8-K.  None.


                                   SIGNATURES

         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.

                                          CENTRAL ILLINOIS PUBLIC
                                             SERVICE COMPANY
                                               (Registrant)


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

Date:  November 15, 1999


                                      -15-