<PAGE>

                                                                    EXHIBIT 10.6

                          GENERAL SERVICES AGREEMENT

                                    Between

                            AMEREN SERVICES COMPANY

                                      and

                        AMEREN ENERGY RESOURCES COMPANY

     THIS AGREEMENT, made and entered into this Third day of September 1999, by
and between the following Parties:  AMEREN SERVICES COMPANY (hereinafter
sometimes referred to as "Service Company"), a Missouri corporation; and Ameren
Energy Resources Company (hereinafter sometimes referred to as "Client
Company");

                                 WITNESSETH:

     WHEREAS, Client Company, and its other subsidiaries, desire to enter into
this agreement providing for the performance by Service Company for the Client
Company of certain services more particularly set forth herein; and

     WHEREAS, Service Company is organized, staffed and equipped and has filed
with the Securities and Exchange Commission ("the SEC") to be a subsidiary
service company under Section 13 of the Public Utilities Holding Company Act of
1935 (the "Act") to render to Ameren Corporation, and other subsidiaries of
Ameren Corporation, certain services as herein provided; and

     WHEREAS, to maximize efficiency, and to achieve merger related savings, the
Client Company desires to avail itself of the advisory, professional, technical
and other services of persons employed or to be retained by Service Company, and
to compensate Service Company appropriately for such services,
<PAGE>

     NOW, THEREFORE, in consideration of the premises and of the mutual
agreements herein, the parties hereto agree as follows:

Section 1. Agreement to Furnish Services
----------------------------------------

     Service Company agrees to furnish to Client Company and its subsidiaries,
if any, upon the terms and conditions herein provided, the services hereinafter
referred to and described in Section 2, at such times, for such period and in
such manner as Client Company may from time to time request.  Service Company
will keep itself and its personnel available and competent to render to Client
Company such services so long as it is authorized so to do by the appropriate
federal and state regulatory agencies.

Section 2.  Services to be Performed
------------------------------------

     The services to be provided by Service Company hereunder may, upon request,
include the services as set out in Schedule 1, attached hereto and made a part
hereof.  A revised Schedule 1 will be provided on an annual basis.

     In addition to the Services set out in Schedule 1, Service Company shall
render advice and assistance in connection with such other matters as Client
Company may request and Service Company determines it is able to perform with
respect to Client Company's business and operations.

Section 3.  Compensation of Service Company
            -------------------------------

     As compensation for such services rendered to it by Service Company, Client
Company hereby agrees to pay to Service Company the cost of such services,
computed in accordance with applicable rules and regulations (including, but not
limited to, Rules 90 and 91) under the Act and appropriate accounting standards.
<PAGE>

     Compensation to be paid by Client Company shall include direct charges and
Client Company's fairly allocated pro rata share of certain of Service Company's
costs, determined as set out on Schedule 2, attached hereto and made a part
hereof.

Section 4.  Securities and Exchange Commission Rules
            ----------------------------------------

     It is the intent of the Parties that the determination of the costs as used
in this Agreement shall be consistent with, and in compliance with the rules and
regulations of the SEC, as they now read or hereafter may be modified by the
Commission.

Section 5.  Service Requests
            ----------------

     Services will be performed in accordance with a Service Request system,
consisting of work orders established to capture the various types of costs
incurred by Service Company.  Costs will be charged to the appropriate service
requests, which will then be the basis for the billing of costs to Client
Company.

Section 6.  Payment
            -------

     Payment shall be by making remittance of the amount billed or by making
appropriate accounting entries on the books of the companies.

     Payment shall be accomplished on a monthly basis, and remittance or
accounting entries shall be completed within 60 days of billing.
<PAGE>

Section 7.  Ameren Corporation
            ------------------

     Except as authorized by rule, regulation, or order of the Securities and
Exchange Commission, nothing in this Agreement shall be read to permit Ameren
Corporation, or any person employed by or acting for Ameren Corporation, to
provide services for other Parties, or any companies associated with said
Parties.

Section 8.  Client Company
            --------------

     Except as limited by Section 7, nothing in this Agreement shall be read to
prohibit Client Company or its subsidiaries from furnishing to other Ameren
companies or their subsidiaries services herein referred to, under the same
conditions and terms as set out for Service Company.

Section 9.  Effective Date and Termination
            ------------------------------

     This Agreement is executed subject to the consent and approval of all
applicable regulatory agencies, and if so approved in its entirety, shall become
effective as of the date stated below, and shall remain in effect from said date
unless terminated by mutual agreement or by any Party giving at least sixty
days' written notice to the other Parties prior to the beginning of any calendar
year, each Party fully reserving the right to so terminate the Agreement.

     This Agreement may also be terminated to the extent that performance may
conflict with any rule, regulation or order of the Securities and Exchange
Commission adopted before or after the making of this Agreement.

Section 10.  Assignment
             ----------
<PAGE>

     This Agreement and the rights hereunder may not be assigned without the
mutual written consent of all Parties hereto.

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

                                AMEREN SERVICES COMPANY

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

                                Title  Treasurer
                                      ----------------------------

ATTEST:

By /s/ Ronald K. Evans
   ----------------------

Title Assistant Secretary
      -------------------
                                AMEREN ENERGY RESOURCES COMPANY

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

                                Title President
                                      ----------------------------

ATTEST:

By /s/ Ronald K. Evans
   ----------------------

Title Assistant Secretary
      -------------------
<PAGE>

                                                                      Schedule 1
                                                                     Page 1 of 9

                       DESCRIPTION OF EXPECTED SERVICES
                     TO BE PROVIDED BY AMEREN SERVICES AND
                    EXPECTED DIRECT COST ALLOCATION FACTORS


Description of Expected Services to be Provided

A description of the expected services to be provided by Ameren Services is
detailed below.  Identifiable costs for all of the functional organizations
listed below will be directly charged to Ameren Corporation and its
subsidiaries, whenever possible.

For costs that cannot be directly assigned or distributed, the expected direct
cost allocation factors are reflected below for each Ameren Services department.

     a)  Building Service

     Description - Provide facility management services for owned and leased
     facilities, excluding power plants.  To the extent that leasing
     arrangements are established between Ameren Services and/or Ameren
     Corporation and its subsidiaries, lease costs will include rent for space
     occupied and applicable services, such as operation and maintenance of
     structures, capital improvements, interior space planning, security and
     janitorial.  As appropriate, lease costs will be allocated based on square
     feet occupied and the allocation factors listed below.

     Expected Allocation Factors - 1) number of employees; 2) operations and
     maintenance labor; 3) total capitalization; and 4) total assets

     b)  Controller's

     Description - Perform all accounting services necessary to properly
     maintain and report on the books and records of Ameren and its
     subsidiaries.  Provide investor relations services.

     Expected Allocation Factors - 1) composite*; 2) total capitalization; and
     3) total assets

     c)  Corporate Communications

     Description - Develop strategies for advertising and marketing efforts,
     develop employee communication programs, coordinate community relations
     efforts and develop policies
<PAGE>

                                                                      Schedule 1
                                                                     Page 2 of 9

     and procedures for media relations.

     Expected Allocation Factors - 1) composite*; 2) total capitalization; and
     3) total assets

     d)  Corporate Planning

     Description - Provide rate engineering, interchange marketing, resource
     planning and business analysis services.

     Expected Allocation Factors - 1) composite*; 2) kwh sales; 3) peak load
     [electric]; 4) total capitalization; and 5) total assets

     e) Customer Services/Division Support

     Description - Answer customer inquiries pertaining to electric/gas service
     usage and perform credit activities.  Provide technical support relating to
     planning, engineering, constructing and operating the distribution and
     transmission systems.  Provide technical support and maintenance of
     protective relay schemes, station meter work, system testing and data
     acquisition systems.

     Expected Allocation Factors - 1) number of customers; 2) number of
     employees; and 3) operations and maintenance labor

     f)  Economic Development

     Description - Provide community and business development services, as well
     as natural gas development services.  Analyze community and business
     development opportunities.

     Expected Allocation Factors - 1) number of customers; 2) sales [kwh and
     dekatherm]; 3) total capitalization; and 4) total assets

     g)  Energy Supply

     Coordinate the use of the generating, transmission and interconnection
     facilities to provide economical and reliable energy.

     Expected Allocation Factors - 1) kwh sales
<PAGE>

                                                                      Schedule 1
                                                                     Page 3 of 9

     h)  Engineering and Construction

     Description - Provide professional services related to engineering studies,
     design, procurement, planning, building and management of projects.  Study
     technology that may reduce costs of producing, delivering and using
     electricity.

     Expected Allocation Factors - 1) peak load [electric]; 2) generating
     capacity; and 3) construction expenditures

     i)  Environmental Services & Safety

     Description - Perform analysis and advocacy of regulatory and legislative
     issues in the areas of environment, health and safety.  Communicate final
     regulatory requirements to operating groups.  Provide assistance and
     support and compliance review in meeting those requirements.  Oversee
     hazardous substance site investigation and remediation activities.

     Expected Allocation Factors - 1) number of employees; 2) generating
     capacity; 3) operations and maintenance labor; and 4) construction
     expenditures

     j)  Executive

     Description - Provide executive management duties for all applicable
     activities at the department, function and officer levels.

     Expected Allocation Factors - 1) total capitalization; 2) total assets; and
     3) sales [kwh and dekatherm]

     k)  Fossil Fuel Procurement

     Description - Provide resources necessary to procure fuel for the fossil
     power plants and minimize production costs.

     Expected Allocation Factors - 1) kwh sales

     l)  Gas Supply

     Description - Provide gas supply and pipeline capacity procurement and
     management services.  Develop policies, procedures and standards which
     govern the design, construction and operation of the gas systems.
<PAGE>

                                                                      Schedule 1
                                                                     Page 4 of 9

     Expected Allocation Factors - 1) dekatherm sales; 2) gas throughput
     [includes transportation]; and 3) peak load [gas]

     m)  General Counsel

     Description - Provide general legal advice related to all applicable
     activities and legal services in regards to legislative activities,
     regulatory agencies and security matters.  Make regulatory filings,
     maintain minutes of the board of directors, conduct stockholder meetings
     and procure property and casualty insurance bonds.

     Expected Allocation Factors - 1) composite*; 2) total capitalization; and
     3) total assets

     n)  Human Resources

     Description - Administer and negotiate employee benefits including
     pensions, major medical, long-term disability, life insurance, defined
     contribution plans, executive benefit and flexible spending plans.  Provide
     employment services, including required regulatory reporting and
     maintenance of personnel records.  Provide employee training and
     communications services.

     Expected Allocation Factors - 1) number of employees; 2) total
     capitalization; 3) total assets; and 4) operation and maintenance labor

     o)  Industrial Relations

     Description - Negotiate, represent and administer provisions of labor
     agreements applicable to unions representing union employees.

     Expected Allocation Factors - 1) number of employees; and 2) operation and
     maintenance labor

     p)  Information Services

     Description - Provide for the development and operation of computer
     software, telecommunications and other equipment used to conduct business
     and engineering activities.  Maintain all billing records and process
     customer meter readings.

     Expected Allocation Factors - 1) composite*; 2) number of customers; 3)
     number of employees; 4)CPU cycles; and 5) operation and maintenance labor
<PAGE>

                                                                      Schedule 1
                                                                     Page 5 of 9

     q)  Internal Audit

     Description - Audit company operations, perform operational and
     productivity reviews, review justifications for capital projects and
     perform quality assurance reviews.

     Expected Allocation Factors - 1) composite*; 2) number of customers; 3)
     number of employees; and 4) operation and maintenance labor

     r)  Marketing

     Description - Provide marketing services including account management,
     program development, market research and customer energy services.

     Expected Allocation Factors - 1) sales [kwh and dekatherm]; and 2) total
     assets

     s)  Merger Coordination

     Description - Monitor programs to achieve savings, merger costs and
     position reductions as they relate to the implementation plans.

     Expected Allocation Factors - 1) composite*; 2) total capitalization; and
     3) total assets

     t)  Motor Transportation

     Description - Provide engineering, support, and mechanical servicing of
     vehicles, procurement of vehicles and safety and training programs.

     Expected Allocation Factors - 1) number of vehicles

     u)  Purchasing

     Description - Provide procurement of goods and services other than fuel.
     Provide materials inventory management services.

     Expected Allocation Factors - 1) composite*; 2) total assets; and 3)
     construction expenditures
<PAGE>

                                                                      Schedule 1
                                                                     Page 6 of 9

     v)  Real Estate

     Description - Acquire necessary land rights and permits including
     coordination of site selection.  Maintain existing land rights while
     permitting licenses and leases to minimize investment or costs of holding
     property.

     Expected Allocation Factors - 1) composite*; 2) number of customers; and 3)
     total assets

     w)  Stores

     Description - Provide clerical, stenographic, administrative and Electronic
     Data systems support.  Provide engineering support and manage and direct
     stores operations.

     Expected Allocation Factors - 1) composite*

     x)  Tax

     Description - Research and consult on tax issues in connection with
     federal, state and local tax compliance and planning matters, including the
     preparation and filing of returns.

     Expected Allocation Factors - 1) composite*; 2) current tax expense; 3)
     total capitalization; and 4) total assets

     y)  Treasurer's

     Description - Provide treasury operation, mailing, financial planning,
     investments, and executive payroll and pension disbursement services.

     Expected Allocation Factors - 1) composite*; 2) number of customers; 3)
     number of employees; 4) total capitalization; and 5) total assets



*Composite consists of the following three factors (equal weight to each
factor):
     Sales (kwh and dekatherm)
     Number of customers
     Number of employees
<PAGE>

                                                                      Schedule 1
                                                                     Page 7 of 9

Allocation Factors

The following allocation factors will be utilized as outlined above.

Number of Customers - Based on the number of customers (electric and/or gas) at
the end of the most recent calendar year.  The numerator of which is for an
Operating Company and the denominator of which is for all Operating Companies.
This ratio will be determined annually, and/or at such time as may be required
due to a significant change in circumstances.

Sales - Based on the sales volume (kwh and/or dekatherms) for the most recent
calendar year.  The numerator of which is for an Operating Company and the
denominator of which is for all Operating Companies.  This ratio will be
determined annually, and/or at such time as may be required due to a significant
change in circumstances.

Number of Employees - Based on the number of employees (contract and/or non-
contract, or electric operating and/or gas operating) at the end of the most
recent calendar year.  The numerator of which is for an Operating Company or an
affected affiliate company.  The denominator of which is for all Operating
Companies and affected affiliate companies.  This ratio will be determined
annually, and/or at such time as may be required due to a significant change in
circumstances.

Composite - Based on an equal weighting Sales (kwh & dekatherm), Number of
Customers (total), and Number of Employees (total) allocation factors.  The
numerator of which is the simple average of the above three factors for an
Operating Company and the denominator of which is for all Operating Companies.
This ratio will be determined annually and/or at such time as may be required
due to a significant change in circumstances.

Operations & Maintenance Labor - Based on the Operations & Maintenance Labor
(electric and/or gas) for the most recent calendar year.  The numerator of which
is for an Operating Company or an affected affiliate and the denominator of
which is for all Operating Companies and affected affiliate companies.  This
ratio will be determined annually, and/or at such time as may be required due to
a significant change in circumstances.

Revenues - Based on revenues (electric and/or gas) for the most recent calendar
year.  The numerator of which is for an Operating Company or an affected
affiliate company.  The denominator of which is for all Operating Companies
and/or affected affiliate companies.  This ratio will be determined annually, or
at such time as may be required due to a significant change in circumstances.
<PAGE>

                                                                      Schedule 1
                                                                     Page 8 of 9

Total Capitalization - Based on total capitalization (total common stockholder's
equity, preferred stock, and long term debt) at the end of the most recent
calendar year.  The numerator of which is for an Operating Company or an
affected affiliate company. The denominator of which is for all Operating
Companies and affected affiliate companies.  This ratio will be determined
annually, and/or at such time as may be required due to a significant change in
circumstances.

Total Assets - Based on total assets at the end of the most recent calendar
year.  The numerator of which is for an Operating Company or an affected
affiliate company.  The denominator of which is for all Operating Companies and
affected affiliate companies.  This ratio will be determined annually, and/or at
such time as may be required due to a significant change in circumstances.

Construction Expenditures - Based on construction expenditures for the most
recent calendar year.  The numerator of which is for an Operating Company or an
affected affiliate company.  The denominator of which is for all Operating
Companies and affected affiliate companies.  This ratio will be determined
annually, and/or at such time as may be required due to a significant change in
circumstances.

Peak Load (electric) - Based on the highest monthly maximum megawatt load (60-
minute integration) for the most recent calendar year.  The numerator of which
is for an Operating Company and the denominator of which is for all Operating
Companies.  This ratio will be determined annually, and/or at such time as may
be required due to a significant change in circumstances.

Peak Load (gas) - Based on the highest daily send out in therms (excluding
transportation) for the most recent calendar year.  The numerator of which is
for an Operating Company and the denominator of which is for all Operating
Companies.  This ratio will be determined annually, and/or at such time as may
be required due to a significant change in circumstances.

Generating Capacity (nameplate) - Based on installed capacity nameplate ratings
at the end of the most recent calendar year.  The numerator of which is for an
Operating Company and the denominator of which is for all Operating Companies.
This ratio will be determined annually, and/or at such time as may be required
due to a significant change in circumstances.

Gas Throughput - Based on gas throughput in dekatherms (sales and
transportation) for the most recent calendar year.  The numerator of which is
for an Operating Company.  The denominator of which is for all Operating
Companies.  This ratio will be determined annually, and/or at such time as may
be required due to a significant change in circumstances.
<PAGE>

                                                                      Schedule 1
                                                                     Page 9 of 9

CPU Cycles - Based on cpu cycles (by application) for the most recent calendar
year.  The numerator of which is for an Operating Company or an affected
affiliate company.  The denominator of which is for all Operating Companies and
affected affiliate companies.  This ratio will be determined annually, and/or at
such time as may be required due to a significant change in circumstances.

Current Tax Expense - Based on taxes charged (income and other) for the most
recent calendar year.  The numerator of which is for an Operating Company or an
affected affiliate company.  The denominator of which is for all Operating
Companies and affected affiliate companies.  This ratio will be determined
annually, and/or at such time as may be required due to a significant change in
circumstances.

Number of Vehicles - Based on number of vehicles at the end of the most recent
calendar year.  The numerator of which is for an Operating Company and the
denominator of which is for all Operating Companies.  This ratio will be
determined annually, and/or at such time as may be required due to a significant
change in circumstances.

In addition to the allocation factors listed above, appropriate direct
allocations will be made for costs benefiting a single affiliate.  Indirect
allocations will also be made to all affiliates, including non-regulated
companies and Ameren Corporation.

It may be necessary to allocate a percentage of total costs allocated to non-
regulated companies or Ameren Corporation (see below).  This will be done as a
sub-factor of existing allocation factors.   For example, allocating a
percentage of customer service costs to non-regulated companies and allocating
remaining costs based on number of customers.  Also, allocating a percentage of
video presentation costs to Ameren Corporation and allocating remaining costs
based on capitalization.

Non-Regulated - Based on a percentage of total costs allocated to non-regulated
companies when existing allocation methods do not adequately reflect the level
of services or benefits received.  After allocating this percentage of total
costs to non-regulatory company, the remaining costs will be allocated to Ameren
Corporation and/or its subsidiaries, as appropriate, based upon one of the
factors above.

Corporate - Based on a percentage of total costs allocated to Ameren Corporation
(AMC) when existing allocation methods do not adequately reflect the level of
services or benefits received.  After allocating this percentage of total costs
to AMC, the remaining costs will be allocated based upon one of the factors
above.
<PAGE>

                                                                      Schedule 2
                                                                     Page 1 of 1


                                AMEREN SERVICES
                    EXPECTED ALLOCATED DIRECT COST FACTORS



   ALLOCATION NUMBER     DESCRIPTION
   -----------------     -----------

          001A           Composite*
          002A           Number of customers
          002B           Number of gas transportation customers
          002C           Number of electric customers
          002D           Number of gas customers
          002E           % to unregulated company/number of customers
          002F           Number of Customers (Illinois Non-Residential Electric)
          002G           Number of Customers (Illinois Non-residential Gas)
          002H           Number of Customers (Illinois Non-residential)
          003A           Sales (kwh and dekatherm)
          003B           Kwh sales
          003C           Dekatherm sales
          004A           Number of employees
          004B           Number of contract employees
          004C           Number of non-contract employees
          004D           Number of AMS & UEC employees
          005A           O&M labor
          006A           Total revenues
          006B           Electric revenues
          006C           Gas revenues
          007A           Total capitalization
          007B           % to Ameren Corporation/total capitalization
          008A           Total assets
          009A           Construction expenditures
          010A           Peak load (electric)
          010B           Peak load (gas)
          011A           Generating capacity
          012A           Gas throughput (includes transportation)
          013A           CPU cycles - mainframe
          013B           CPU cycles - UNIX
          015A           Current tax expense
          016A           Number of vehicles


*Composite consists of the following three factors (equal weight to each
factor):
          Sales (kwh and dekatherm)
          Number of customers
          Number of employees