<PAGE>

                                                                    EXHIBIT 10.4


                                    AMENDED

                                JOINT DISPATCH

                                   AGREEMENT



                                    Between



                            UNION ELECTRIC COMPANY,

                    CENTRAL ILLINOIS PUBLIC SERVICE COMPANY

                                      AND

                       AMEREN ENERGY GENERATING COMPANY
<PAGE>

                                JOINT DISPATCH

                                   AGREEMENT

                                    Between

                            UNION ELECTRIC COMPANY,

                    CENTRAL ILLINOIS PUBLIC SERVICE COMPANY

                                      AND

                       AMEREN ENERGY GENERATING COMPANY

     THIS AGREEMENT is made and entered into this first day of May, 2000 by and
between UNION ELECTRIC COMPANY ("UE") a Missouri corporation, CENTRAL ILLINOIS
PUBLIC SERVICE COMPANY ("CIPS") an Illinois corporation, and AMEREN ENERGY
GENERATING COMPANY ("GENCO"), an Illinois corporation, referred to collectively
as "Parties" and singularly as "Party," all of whose common stock is wholly
owned by Ameren Corporation, hereinafter called "Parent", a Missouri
corporation.

                                  WITNESSETH:

     WHEREAS, UE and CIPS previously entered into a Joint Dispatch Agreement,
dated December 18, 1995; and

     WHEREAS, UE and CIPS are the owners and operators of electric generation,
transmission and distribution facilities and are engaged in the business of
generating, transmitting, distributing and selling electric energy to the
general public, electric utilities, municipalities and cooperatives; and

     WHEREAS, to maximize efficiency, UE and CIPS operate as an integrated
control area, and economically commit and dispatch the combined Generating
Resources, and economically utilize power and energy available to the Combined
System to transact with other utilities and

                                       1
<PAGE>

wholesale entities in order to operate the Combined System in a reliable,
efficient, and economic manner; and

     WHEREAS, CIPS intends to transfer all of its generating assets to Genco;
and

     NOW, THEREFORE, in consideration of the covenants and premises herein set
forth, the Parties mutually agree as follows:

                                   ARTICLE I

                                  DEFINITIONS

     For the purpose of this agreement, and the Appendices and Service Schedules
which are a part hereof, the following definitions shall apply:

     1.01  After-the-Fact Resource Allocation shall mean a methodology used to
           ----------------------------------
assign the Combined System's Generating Resources and Off-System Power Purchases
to each Party's Load Requirements and to the Combined System's Off-System Sales.
After-the-Fact Resource Allocation shall be run for each calendar day after the
calendar day has transpired.

     1.02  Agent shall mean the entity designated to perform certain
           -----
administrative and coordination functions for the Parties.

     1.03  Agreement shall mean this Joint Dispatch Agreement together with all
           ---------
Appendices and Service  Schedules applying thereto and any amendments made
hereafter.

     1.04  Combined System shall mean the combined Generating Resources and
           ---------------
transmission facilities of the Parties.

     1.05  Control Area shall mean the electric system of UE and CIPS as bounded
           ------------
by interconnection (tie line) metering and telemetry, such that the Generating
Resources are controlled directly to maintain the interchange schedule with
other control areas and to contribute to frequency regulation of the
interconnected system.

                                       2
<PAGE>

     1.06  Electric Utility shall mean any entity engaged in the purchase and
           ----------------
wholesale sale of electric energy.

     1.07  Generating Resources shall mean all power generating facilities owned
           --------------------
by a Party available to meet the capacity and energy needs of the Parties. A
list of the generating facilities and the owning Party for each facility is
included in Appendix 1.

     1.08  Generating Parties shall mean those Parties owning Generating
           ------------------
Resources.

     1.09  Generating Unit shall mean an electric generator, together with all
           ---------------
auxiliary and appurtenant devices and equipment designed to be operated as a
unit for the production of electric power and energy.

     1.10  Incremental Cost shall mean any costs incurred by a Generating Party
           ----------------
solely by reason of its generation of an incremental amount of energy, which may
include but shall not be limited to, costs of fuel, labor, operation,
maintenance, start-up, fuel handling, taxes, regulatory commission charges,
transmission losses and emissions allowances.

     1.11  Load Requirements shall mean the demand and energy which each
           -----------------
Generating Party is obligated to serve pursuant to service territory commitments
and requirements agreements.

     1.12  Net Output shall mean each Generating Party's monthly total of the
           ----------
energy delivered for Load Requirements.

     1.13  Off-System Purchases shall mean purchases from a third party of
           --------------------
energy and/or associated capacity to reduce costs and/or to provide reliability
for the system or as required by law.

     1.14  Off-System Sales shall mean all sales of power and/or energy to third
           ----------------
parties other than Load Requirements.

                                       3
<PAGE>

     1.15  Off-System Sales Margin shall mean the difference between the energy
           -----------------------
revenue collected from Off-System Sales and the energy cost of providing such
sales, as assigned by the After-the-Fact Resource Allocation.

     1.16  Operating Committee shall mean the organization created under this
           -------------------
Agreement to administer its provisions and to undertake the responsibilities set
forth in Article VII hereunder.

     1.17  Service  Schedules shall mean the service schedules attached hereto
           ------------------
and those which later may be agreed to by the Parties and accepted for filing by
the Federal Energy Regulatory Commission ("FERC").

     1.18  Surplus Reserve Ratio shall mean the ratio calculated at the
           ---------------------
beginning of each month of each Generating Party's surplus reserve to the sum of
both Generating Parties' surplus reserve.  Surplus reserve shall be calculated
for each Generating Party in megawatts by computing the sum of the Generating
Party's rated capabilities of its Generating Resources, plus the Generating
Party's own non-firm capacity purchases, less its own non-firm capacity sales,
less megawatts not available due to scheduled maintenance and long-term forced
outages, less 1.15 times the sum of its projected peak demand component of the
Load Requirements for the month, plus its firm capacity sales, less its firm
capacity purchases.

     1.19  System Dispatch shall mean the centralized, economic commitment and
           ---------------
dispatch of the Combined System's Generating Resources and Off-System Purchases.

     1.20  System Energy Transfer shall mean the transfer of electric energy
           ----------------------
from one Party's Generating Resources to the other Generating Party to serve the
other Generating Party's Load Requirements.

                                  ARTICLE II

                               TERM OF AGREEMENT

                                       4
<PAGE>

     2.01  This Agreement shall take effect upon approval by the FERC, and shall
continue in full force and effect until terminated by one or more of the
Parties, such Party(ies) having given at least one year's written notice, but in
no event shall this Agreement be terminated prior to December 31, 2004.

     2.02  This Agreement will be reviewed periodically by the Operating
Committee to determine whether revisions are necessary or appropriate.

                                  ARTICLE III

                                    PURPOSE

     The purpose of this Agreement is to provide the contractual basis for
coordinated operation of the Combined System to achieve economies consistent
with the provision of reliable electric service and an equitable sharing of the
benefits and costs of such coordinated operation among the Parties.

                                  ARTICLE IV

                                     AGENT

     4.01  Responsibility of the Agent
           ---------------------------

           As soon as practicable after this Agreement becomes effective, the
Parties shall designate an Agent for the purpose of:

     a)  coordinating the System Dispatch;

     b)  maintaining the reliability of the Combined System through monitoring
and security assessments;

     c)  arranging and scheduling Off-System Purchases;

     d)  coordinating the provision of transmission service;

                                       5
<PAGE>

     e)  the development of certain bills and billing related information;

     f)  operation and maintenance of a central control center to achieve these
purposes; and

     g)  other such activities and duties as may be necessary or as assigned by
the Operating Committee.

     4.02  Expenses
           --------

           All expenses incurred by the Agent in the performance of its
responsibilities shall be settled in accordance with the arrangements made by
the Parties for compensation for services provided between or on behalf of the
Parties.

                                   ARTICLE V

                             COORDINATED OPERATION

     5.01  Operation of the Combined System
           --------------------------------

           The Agent shall administer the System Dispatch of the Combined System
in order to economically meet the Generating Parties' combined Load Requirements
and Off-System Sales obligations, through the economic commitment and dispatch
of the Combined System's Generating Resources and Off-System Purchases,
consistent with reliable operation of the interconnected system as defined in
Article XI and the Generating Parties' supply obligations.  The Agent shall
engage in arranging and scheduling economical Off-System Purchases, as a single
Control Area, utilizing the available transmission resources of the Combined
System.

     5.02  Communications and Other Facilities
           -----------------------------------

           The Parties shall provide communications, metering and other
facilities necessary for the metering and control of the Generating Resources
and interconnected transmission facilities. Each Party shall be responsible for
any expenses it incurs for the installation, operation

                                       6
<PAGE>

and maintenance of facilities at its own Generating Units and interconnected
transmission facilities. Any expenses incurred due to facilities required at or
for the central control center to operate the Combined System shall be settled
in accordance with the arrangements made by the Parties for compensation for
services provided between and on behalf of the Parties.

                                  ARTICLE VI

                       ASSIGNMENT OF COSTS AND BENEFITS

                           OF COORDINATED OPERATIONS

     6.01  Fixed Costs of Existing Generating Resources
           --------------------------------------------

           For all purposes relevant to this Agreement, each Generating Party
will retain all costs not collected pursuant to Section 6.07 of its existing
Generating Resources that are listed in Appendix 1 attached hereto.  Generating
unit retirements or permanent derates will be assigned to the Generating Party
owning the Generating Unit.

     6.02  Environmental Costs of Existing Generating Resources
           ----------------------------------------------------

           The cost of environmental compliance (e.g., compliance with the Clean
Air Act Amendments of 1990) associated with the existing Generating Resources
will be borne by the Generating Party that owns the unit.  The Generating
Parties will maintain and account for each unit's emissions allowance
allocation.

     6.03  Demand Charges From Existing Off-System Purchases
           -------------------------------------------------

           Demand Charges from existing Off-System Purchases agreed to as of the
effective date of this Amended Agreement, shall remain the responsibility of the
Generating Party contracting for the purchase.

     6.04  Demand Charges From New Off-System Purchases
           --------------------------------------------

                                       7
<PAGE>

          Demand Charges Associated With New Off-System Purchases made to enable
the Agent to reliably and economically meet the Generating Parties' combined
Load Requirements shall be assigned to the Generating Parties based on the ratio
of the demand component (the one hour integrated peak demand) of the Load
Requirements of the Generating Parties for the appropriate time period.

          Demand charges associated with new Off-System Purchases made to enable
the Agent to make new Off-System Sales or to supply existing Off-System Sales
shall be deducted from the demand charge revenue collected from the Off-System
Sales.  The net amount shall be allocated to the Parties pursuant to Sections
6.05 and 6.06.

          This section applies only to demand charges associated with new Off-
System Purchases made for System Dispatch and not to purchases made by a Party
for purposes of maintaining adequate planning reserve margin, which
responsibility shall remain with each Party.

     6.05 Demand Charges From Existing Off-System Sales
          ---------------------------------------------

          Demand charge revenues collected for existing Off-System Sales, as
agreed to as of the effective date of this Agreement, shall remain with the
Party contracting for the sale.

     6.06 Demand Charges From New Off-System Sales
          ----------------------------------------

          Demand charge revenues collected for new Off-System Sales shall be
reduced by any demand charges from Off-System Purchases, if any, dedicated to
supply the sale, pursuant to Section 6.04.  On a monthly basis, the net amount
of revenue shall be allocated to the Generating Parties based on the projected
monthly Surplus Reserve Ratio.

     6.07 Assignment of Energy and Costs From System Dispatch
          ---------------------------------------------------

          The Agent shall use After-the-Fact Resource Allocation to assign the
energy resources used by the Parties in coordinated operation to each Generating
Party and the Off-

                                       8
<PAGE>

System Sales. The After-the-Fact Resource Allocation shall be applied consistent
with the following principles:

          a)  Energy from the lowest Incremental Cost generation from each
     Generating Party's own Generating Resources shall first be assigned to its
     own Load Requirements.

          b)  Energy available from Off-System Purchases made by one of the
     Generating Parties, including existing Off-System Purchases, shall be
     assigned to the Generating Party who contracted for the purchase, when it
     is economical.  Any energy from Off-System Purchases made by one of the
     Generating Parties, which the After-the-Fact Resource Allocation does not
     assign economically to any Generating Party, shall be assigned to the
     Generating Party who contracted for the purchase.  The cost of energy
     assigned shall be the actual cost of the energy component of the Off-System
     Purchase.

          c)  Energy from Generating Resources which are not economical to be
     operated per System Dispatch but are utilized due to operating constraints
     shall be allocated to the Generating Party owning the generating unit(s),
     unless the other Generating Party's Load Requirements or operating
     conditions are clearly identified as the reason for the generation, in
     which case the energy is assigned to the other Generating Party as a System
     Energy Transfer.

          d)  Energy from other Off-System Purchases will be assigned to the
     Generating Parties based on the economics of the purchase.  Where a new
     Off-System Purchase would be economic for both Generating Parties' Load
     Requirements over the appropriate time period, or is not assigned
     economically to any Generating Party, the energy from the Off-System
     Purchase shall be shared between the Generating Parties based on the ratio
     of the Load Requirements of the Generating Parties.  The cost of the

                                       9
<PAGE>

     energy assigned to each Generating Party shall be the actual cost of the
     energy component of the Off-System Purchase.

          e)  Energy from one Party's Generating Resources utilized by the other
     Generating Party to serve that Party's Load Requirements shall be called
     System Energy Transfer.  Where After-the-Fact Resource Allocation
     identifies a System Energy Transfer as the source to supply one Generating
     Party's Load Requirements, the determination of cost for the System Energy
     Transfer and reimbursement shall be made pursuant to Service Schedule A,
     System Energy Transfer.

          f)  Energy from Off-System Purchases may be assigned by the After-the-
     Fact Resource Allocation or designated by the Agent to be used to supply
     Off-System Sales.  The actual cost of the Off-System Purchase shall be
     deducted from the energy revenue collected from the Off-System Sale.  The
     net amount shall be included in the calculation of the Off-System Sales
     Margin.

          g)  Energy from the Generating Parties' Generating Resources which is
     not assigned to any Party's Load Requirements shall be assigned to Off-
     System Sales according to established priorities.  The cost of the energy
     assigned to Off-System Sales shall be the Incremental Cost of the
     Generating Resources used to supply the sale.  This cost shall be deducted
     from the energy revenue collected from the Off-System Sale.  The net amount
     shall be included in the calculation of Off-System Sales Margin.

     6.08 Distribution of the Off-System Sales Margin
          -------------------------------------------

          The Off-System Sales Margin shall be distributed to the Parties
pursuant to Service Schedule B, Distribution of Off-System Sales Margin.

                                  ARTICLE VII

                                       10
<PAGE>

                  ASSIGNMENT OF TRANSMISSION SERVICE REVENUES

     7.01  Revenue From Existing Firm Transmission Service Agreements
           ----------------------------------------------------------

           Revenue from existing firm transmission service agreements, agreed to
as of the effective date of this Agreement, shall remain with the Party
contracting for the service.  Should an entity receiving service under an
existing firm transmission service agreement subsequently take service under the
Combined System's Network or Point-to-Point Transmission Service Tariffs, the
revenue collected from that service shall be shared between the Parties pursuant
to Section 7.03.

     7.02  Revenue From Existing Non-Firm Transmission Service Agreements
           --------------------------------------------------------------

           Revenue from existing non-firm transmission service agreements,
agreed to as of the effective date of this Agreement, shall be shared between
the Parties pursuant to Section 7.03.

     7.03  Revenue From the Combined System's Network and Point-to-Point
           -------------------------------------------------------------
           Transmission Service Tariffs
           ----------------------------

           Revenue from the Combined System's Network and Point-to-Point
     Transmission Service Tariff ("Tariff"), imputed transmission revenues from
     Third Party Sales (as that term is defined in the Tariff) by Ameren and any
     other applicable transmission service revenues shall first be assigned to
     the Parties to reimburse each Party for the cost of any direct assignment
     facilities or distribution facilities included in the transmission service
     revenues.  The transmission service revenues shall then be used to
     reimburse either or both of the Parties for any incremental expenses
     incurred to provide the transmission service, which may include, but shall
     not be limited to, costs of facility additions, modifications or
     improvements, uneconomic dispatch costs, losses, and system study costs.
     The revenue remaining shall be assigned to the Parties in proportion to
     each

                                       11
<PAGE>

     Party's Transmission Plant investment relative to the total Transmission
     Plant investment included in the rate calculation in the Tariffs.

                                 ARTICLE VIII

               COMPOSITION AND DUTIES OF THE OPERATING COMMITTEE

     8.01  Operating Committee
           -------------------

           An Operating Committee shall be the administrative organization of
this Agreement and shall consist of six persons, with two members designated by
each Party.

     8.02  Officers of the Operating Committee
           -----------------------------------

           The Operating Committee shall have the following officers with duties
as designated:

           a)  Chairman - The Chairman shall issue calls for and shall preside
               --------
     at meetings of the Operating Committee. The Chairman shall have
     responsibility for the general coordination of the Operating Committee
     functions among the members.

           b)  Vice Chairman - The Vice Chairman shall perform the duties of the
               -------------
     Chairman in the Chairman's absence or incapacity.

           The Chairman and Vice Chairman shall be appointed from the members of
the Operating Committee.  The initial Chairman shall be from UE and the initial
Vice Chairman from Genco, with these Parties alternating those positions
thereafter.  A new Chairman and Vice-Chairman shall be designated by the Parties
at the first meeting held in each odd-numbered calendar year and shall take
office immediately upon being appointed.

     8.03  Meeting Dates
           -------------

                                       12
<PAGE>

           The Operating Committee shall hold meetings at such times as is
appropriate and at any time upon the request of a member of the Operating
Committee, but at least once per calendar year.  Minutes of each Operating
Committee meeting shall be prepared and maintained.

     8.04  Decisions
           ---------

           All decisions of the Operating Committee shall be by a majority vote
of the members present or voting by proxy at the meeting at which the vote is
taken.

     8.05  Duties
           ------

           The Operating Committee shall have the following duties, unless such
duties are otherwise assigned by a vote of the Operating Committee to the Agent,
in which case the Agent shall perform such duties:

           a)  Be responsible for the day-to-day administration of this
     Agreement and the development of any amendments thereto.

           b)  Review and recommend additional duties and responsibilities for
     the Agent and review and recommend changes to the procedures for System
     Dispatch and interchange coordination.

           c)  Monitor the adequacy of reserves for the Parties and the Combined
     System.

           d)  Provide coordination of maintenance schedules for major
     Generating Resources.

           e)  Provide coordination for other matters not specifically provided
     herein which the Parties agree are necessary to operate the Combined System
     economically.

     8.06  Expenses of Committee
           ---------------------

           Each Party shall pay the expenses of its representatives on the
Operating Committee.

                                       13
<PAGE>

                                  ARTICLE IX

                              BILLING PROCEDURES

     9.01  Records
           -------

           The Agent shall maintain such records as may be necessary to
determine the assignment of costs and benefits of coordinated operations
pursuant to Article VI of this Agreement. Such records shall be made available
to the Parties upon request.

     9.02  Monthly Statements
           ------------------

           As promptly as practicable after the end of each calendar month, the
Agent shall prepare a statement setting forth the monthly summary of the costs
and revenues allocated or assigned to the Parties in sufficient detail as may be
needed for settlements under the provisions of this Agreement.

           In months where more than one Party has made System Energy Transfers,
only the net cost of the System Energy Transfers need be reflected in the
statement.

     9.03  Billings and Payments
           ---------------------

           The Agent shall handle all billing between the Parties and other
entities engaging in Off-System Purchases with the Parties.  In addition to any
demand charges or other charges due to one of the Parties from another Party
pursuant to agreements other than this Joint Dispatch Agreement, the Agent shall
also net bill the Parties, by debiting the Parties as appropriate, and pursuant
to Article VI, for:

           a)  Demand and energy charges for Off-System Purchases, and

           b)  the cost of System Energy Transfers where the Party was the
     recipient;

     and crediting the Parties, as appropriate, and pursuant to Article VI and
VII, for:

           a)  the cost of System Energy Transfers where the Party was the
     supplier, and

                                       14
<PAGE>

           b)  Transmission service revenues;

     and shall determine the billing and payment under the System Support
Agreement.

           All bills will be based on net amounts owed.  Payment shall be by
making remittance of the amount billed or by making appropriate accounting
entries on the books of the Parties.

     9.04  Taxes
           -----

           Should any federal, state, or local tax, in addition to such taxes as
may now exist, be levied upon the electric power, energy, or service to be
provided in connection with this Agreement, or upon the provider of service as
measured by the power, energy, or service, or the revenue therefrom, such
additional tax shall be included in the net billing as described in Section
9.03.

                                   ARTICLE X

                                 FORCE MAJEURE

     In case a Party should be delayed in or prevented from performing or
carrying out any of the agreements, covenants, or obligations made by or imposed
upon the Parties by this Agreement, either in whole or in part, by reason of or
through strike, work stoppage of labor, failure of contractors or suppliers of
materials (including fuel), failure of equipment, environmental restrictions,
riot, fire, flood, ice, invasion, civil war, commotion, insurrection, military
or usurped power, order of any Court granted in any bona fide adverse legal
proceedings or action, or of any civil or military authority either de facto or
de jure, explosion, Act of God or the public enemies, or any cause reasonably
beyond its control and not attributable to its neglect; then, and in such case
or cases, such Party shall not be liable to the other Party for or on account of
any loss, damage, injury, or expense resulting from or arising out of such delay
or prevention;

                                       15
<PAGE>

provided, however, that the Party suffering such delay or prevention shall use
due diligence to attempt to remove the cause or causes thereof; and provided,
further, that no Party shall be required by the foregoing provisions to add to,
modify, or upgrade any facilities, or to settle a strike or labor dispute except
when, according to its own best judgment, such action seems advisable.

                                  ARTICLE XI

                              INDUSTRY STANDARDS

     The Parties agree to conform to all applicable NERC and regional
reliability council principles, guides, criteria, and standards and industry
standard practices and conventions of reliable system operations.

                                  ARTICLE XII

                                    GENERAL

     12.01  No Third Party Beneficiaries
            ----------------------------

            This Agreement is not intended to and shall not create rights of any
character whatsoever in favor of any person, corporation, association, entity or
power supplier, other than the Parties, and the obligations herein assumed by
the Parties are solely for the use and benefit of said Parties.  Nothing herein
contained shall be construed as permitting or vesting, or attempting to permit
or vest, in any person, corporation, association, entity or power supplier,
other than the Parties, any rights hereunder or in any of the electric
facilities owned by said Parties or the use thereof except as may otherwise be
specified herein.

     12.02  Waivers
            -------

                                       16
<PAGE>

            Any waiver at any time by a Party of its right with respect to a
default under this Agreement, or with respect to any other matter arising in
connection with this Agreement, shall not be deemed a waiver with respect to any
subsequent default or matter.  Any delay, short of the statutory period of
limitation, in asserting or enforcing any right under this Agreement, shall not
be deemed a waiver of such right.

     12.03  Successors and Assigns
            ----------------------

            This Agreement shall inure to the benefit of and be binding upon the
Parties only, and their respective successors and assigns, and shall not be
assignable by any Party without the written consent of the other Parties except
to a successor in the operation of its properties by reason of a merger,
consolidation, sale or foreclosure where substantially all such properties are
acquired by or merged with such a successor.

     12.04  Liability and Indemnification
            -----------------------------

            Subject to any applicable state or federal law which may
specifically restrict limitations on liability, each Party shall release,
indemnify, and hold harmless the other Parties, their directors, officers, and
employees from and against any and all liability for loss, damage, or expense
alleged to arise from, or incidental to injury to persons and/or damage to
property in connection with its facilities or the production or transmission of
electric energy by or through such facilities, or related to performance or
nonperformance of this Agreement, including any negligence arising hereunder. In
no event shall either Party be liable to the other Parties for any indirect,
special, incidental, or consequential damages with respect to any claim arising
out of this Agreement.

     12.05  Governing Law
            -------------

            The validity, interpretation and performance of this Agreement and
each of its provisions shall be governed by the applicable laws of the State of
Missouri.

                                       17
<PAGE>

     12.06  Section Headings
            ----------------

            The descriptive headings of the Articles and sections of this
Agreement are used for convenience only, and shall not modify or restrict any of
the terms and provisions thereof.

     12.07  Notice
            ------

            Any notice or demand for performance required or permitted under any
of the provisions of this Agreement shall be deemed to have been given on the
date such notice, in writing, is deposited in the U.S. mail, postage prepaid,
certified or registered mail, addressed to:

            UNION ELECTRIC COMPANY
            Vice President
            P.0. Box 149, MC 1400
            St. Louis, Missouri 63166

     And to:

            CENTRAL ILLINOIS PUBLIC SERVICE COMPANY
            Vice President
            607 East Adams Street
            Springfield, Illinois 62739

     And to:

            AMEREN ENERGY GENERATING COMPANY
            President
            One Ameren Plaza
            1901 Chouteau Avenue
            St. Louis, MO 63103

     as the case may be; or in such other form or to such other address as
either Party shall stipulate.

                                 ARTICLE XIII

                              REGULATORY APPROVAL

     13.01  Regulatory Authorization
            ------------------------

                                       18
<PAGE>

            This Agreement shall be subject to the approval of the regulatory
agencies having jurisdiction.  In the event that this Agreement is not accepted
in its entirety by all such agencies, any Party may terminate this Agreement
immediately.

     13.02  Changes
            -------

            It is contemplated by the Parties that it may be appropriate from
time to time to change, amend, modify or supplement this Agreement or the
Schedules which are attached to this Agreement to reflect changes in operating
practices or costs of operations or for other reasons. This Agreement may be
changed, amended, modified or supplemented by an instrument in writing executed
by the Parties.

     IN WITNESS WHEREOF, the Parties have caused this Agreement to be executed
and attested by their duly authorized officers on the day and year first above
written.

                                       UNION ELECTRIC COMPANY

                                       By /s/ Jerre E. Birdsong
                                          -----------------------------
                                             Treasurer

ATTEST:

/s/ Ronald K. Evans
--------------------------
Assistant Secretary

                                       CENTRAL ILLINOIS PUBLIC
                                       SERVICE COMPANY

                                       By /s/ Gary L. Rainwater
                                          ---------------------------
                                             President

ATTEST:

/s/ Ronald K. Evans
--------------------------
Assistant Secretary

                                       AMEREN ENERGY
                                       GENERATING COMPANY

                                       By /s/ R. Alan Kelley
                                          ---------------------------
                                             Senior Vice President

ATTEST:

/s/ Ronald K. Evans
--------------------------
Assistant Secretary

                                       19
<PAGE>

                              SERVICE SCHEDULE A

                            SYSTEM ENERGY TRANSFER

Al -- Duration  This Service Schedule A shall become effective and binding when
--------------
the Joint Dispatch Agreement becomes effective, and shall continue in full force
and effect throughout the duration of such Agreement.  This Service Schedule A
is a part of the Agreement and, as such, the use of terms in this Service
Schedule A that are defined in the Agreement shall have the same respective
meanings as set forth in the Agreement.

A2 -- Applicability  In accordance with the terms of Articles V and VI of the
-------------------
Agreement, the Combined System's Generating Resources shall be centrally
dispatched on an economic dispatch basis which may result in the transfer of
electric energy from one Party's Generating Resources to another Party to serve
such other Party's Load Requirements, herein called "System Energy Transfers."

A3 -- Compensation  Charges for System Energy Transfer shall be the Incremental
------------------
Cost of the Generating Resources supplying the energy.

                              UNION ELECTRIC COMPANY

                              By /s/ Jerre E. Birdsong
                                 ----------------------------------
                                    Treasurer

                              CENTRAL ILLINOIS PUBLIC
                              SERVICE COMPANY

                              By /s/ Gary L. Rainwater
                                 ----------------------------------
                                    President

                              AMEREN ENERGY
                              GENERATING COMPANY

                              By /s/ R. Alan Kelley
                                 ----------------------------------
                                    Senior Vice President

                                       1
<PAGE>

                              SERVICE SCHEDULE B

                    DISTRIBUTION OF OFF-SYSTEM SALES MARGIN

                        Under Joint Dispatch Agreement

                        Between Union Electric Company,

                    Central Illinois Public Service Company

                                      and

                       Ameren Energy Generating Company

B1 -- Duration  This service Schedule B shall become effective and binding when
--------------
the Joint Dispatch Agreement becomes effective, and shall continue in full force
and effect, throughout the duration of such Agreement.  This Service Schedule B
is a part of the Agreement and, as such, the use of terms in this Service
Schedule B that are defined in the Agreement shall have the same respective
meanings as set forth in the Agreement.

B2 -- Applicability  In accordance with the terms of Articles V and VI of the
-------------------
Agreement, the Combined System shall be centrally dispatched on an economic
dispatch basis and shall engage in economical Off-System Purchases and Off-
System Sales as a single Control Area.  The difference between the energy
revenue collected from Off-System Sales and the costs of providing such sales,
herein called Off-System Sales Margin, is to be distributed between the
Generating Parties.  This Service Schedule defines the formula for distribution.

B3 -- Distribution Formula  The monthly distribution ratio for each Party for
--------------------------
the Off-System Sales Margin shall be the Generating Party's Net Output divided
by the sum of the Parties' Net Output.  The amount of Off-System Sales Margin
distributed to each Generating Party shall be the Generating Party's monthly
distribution ratio times the Off-System Sale Margin.



                                       1
<PAGE>

                              UNION ELECTRIC COMPANY


                              By /s/ Jerre E. Birdsong
                                 -----------------------------
                                    Treasurer

                              AMEREN ENERGY
                              GENERATING COMPANY

                              By /s/ R. Alan Kelley
                                 -----------------------------
                                    Senior Vice President

                                       2
<PAGE>

                              SERVICE SCHEDULE C

                   RECOVERY OF INCREMENTAL COSTS RELATING TO

                             EMISSIONS ALLOWANCES

                        Under Joint Dispatch Agreement

       between Union Electric Company, Ameren Energy Generating Company

                  and Central Illinois Public Service Company


C1 - Duration.  This Service Schedule C shall become effective and binding when
--------------
the Joint Dispatch Agreement becomes effective, and shall continue in full force
and effect throughout the duration of such Agreement.  This Service Schedule C
is a part of the Agreement and, as such, the use of terms in this Service
Schedule C that are defined in the Agreement shall have the same respective
meanings as set forth in the Agreement.

C2 - Applicability.  In accordance with the terms of Articles V and VI of the
-------------------
Agreement, the Combined System shall be centrally dispatched on an economic
dispatch basis and shall engage in economical Off-System Purchases as a single
Control Area.  The cost of the energy from the Parties' Generating Resources to
supply System Energy Transfer is the Incremental Cost of the energy which may
include emissions allowance cost.  This Service Schedule C defines the
methodology for determining the emissions allowance cost.

C3 -- Emissions Allowance Recovery Mechanism.  The emissions allowance cost used
---------------------------------------------
in the computation of Incremental Cost shall be the replacement cost of
emissions allowances.  The emissions allowance replacement cost will be the
"Monthly Price Index" published by Cantor Fitzgerald Environmental Brokerage
Service by the twenty-fifth day of the month prior to the month the transaction
occurs.  The Parties will use the Cantor Fitzgerald index unless one or both of
the Parties is involved in the actual purchase or sale of allowances wherein it
may choose at its

                                       1
<PAGE>

option to use the price of its own transactions, such transactions to have a
minimum allowance quantity of 1,000 allowances. Although the Parties have
designated Cantor Fitzgerald as the index to be used in establishing emissions
allowance cost, the Parties will continue to evaluate other market indicators.
The Parties may in the future designate another index to serve as the
incremental price indicator.

The allowance replacement cost, in $/SO//2// ton, will be used to calculate a
Generating Unit's incremental SO//2// cost as described below.  The incremental
SO//2// cost of operating an affected unit will be calculated using three
components -- the allowance replacement cost, the unit's incremental heat rate
and the SO//2// rate of the fuel used at the unit.

                           AC x HR x SR
                    EC =  _____________
                             2 x 10/6/
Where:  EC = Total Incremental SO//2// Cost ($/Mwh)

          AC = Allowance Replacement Cost ($/SO//2// Ton)

          HR = Incremental Heat Rate (Btu/Kwh)

          SR = SO//2// Rate for Fuel (Lbs of SO//2///MMbtu)

The incremental emissions cost (EC) will be used to dispatch generating units,
make Off-System Purchase decisions, and price System Energy Transfers, pursuant
to this Agreement.  The Generating Unit used to compute the emissions allowance
amount will be the same unit that is used to calculate the Incremental Cost.

                                       2
<PAGE>

Either Party will have the option to pay the allowance replacement cost or
provide equivalent emissions allowances.  Cash payment will be due in accordance
with the terms and conditions of this Agreement.  If a Party elects to provide
emissions allowances, the equivalent emissions allowances will be calculated as
follows:

                              TEC
          Allowances Due =  _______
                              AC

                                       3
<PAGE>

                                  APPENDIX 1

                             GENERATING RESOURCES

Union Electric Company

     Labadie Plant - Franklin County, Mo.

     Meramec Plant - St. Louis County, Mo.

     Rush Island Plant - Jefferson County, Mo.

     Sioux Plant - St. Charles County, Mo.

     Venice Plant - Illinois

     Keokuk Plant - Iowa

     Osage Plant - Lakeside, Mo.

     Taum Sauk Plant - Reynolds County, Mo.

     Callaway Nuclear Plant - Callaway County, Mo.

     Various Combustion Turbine Units - Missouri

Ameren Energy Generating Company

     Newton Plant - Jasper County, Il.

     Coffeen Plant - Montgomery County, Il.

     Meredosia Plant - Morgan County, Il.

     Hutsonville Plant - Crawford County, Il.

     Grand Tower Plant - Jackson County, Il.

     Combustion Turbine Units - specific list to be developed.

                                       4