<PAGE>

                                                                   EXHIBIT 10.34

                             AMENDATORY AGREEMENT

     This Amendment is entered into this 1/st/ day of March, 2000, between
Southern Natural Gas Company ("Company") and Atlanta Gas Light Company
("Shipper").

                                  WITNESSETH

     WHEREAS, Company and Shipper are parties to Firm Transportation Service
Agreement(s) (collectively referred to as "Contract FSNG269") for the following
Transportation Demands and effective dates under the indicated Rate Schedule(s)
of Company's FERC Gas Tariff, Seventh Revised Volume No. 1:

-------------------------------------------------------------------------
Original    Package     Rate     Transportation   Effective   Primary
Contract    Number    Schedule       Demand         Date        Term
Number                              (Mcf/day)
-------------------------------------------------------------------------
904480        1         FT           5,173        11/1/1994   2/28/2001
-------------------------------------------------------------------------
904481        1       FT-NN          6,764        11/1/1994   2/28/2001
-------------------------------------------------------------------------

     WHEREAS, Company and Shipper are also parties to a Firm Contract Storage
Service Agreement (referred to as "Contract SSNG183") for the following Maximum
Storage Quantity ("MSQ") and Maximum Daily Withdrawal Quantity ("MDWQ") for the
effective dates shown under Rate Schedule CSS of Company's FERC Gas Tariff,
Seventh Revised Volume No. 1:

-------------------------------------------------------------------
Original    Package      MSQ       MDWQ       Effective   Primary
Contract    Number    (Mcf/day)  (Mcf/day)      Date        Term
Number
-------------------------------------------------------------------
S20140        1        334,977     6,764      10/30/1998  2/29/2001
-------------------------------------------------------------------

     WHEREAS, Shipper has chosen to support Company's rate case settlement filed
with the Federal Energy Regulatory Commission ("FERC") on March 10, 2000, in
Docket Nos. RP99-496-000 and RP99-496-001 ("Settlement") in order to receive the
benefit of the lower settlement rates effective March 1, 2000, and the other
benefits set forth in the settlement, in exchange for contract extensions of the
above-referenced firm contract(s) for the period(s) specified herein;

     NOW, THEREFORE, in consideration of the premises and the mutual benefits
and covenants contained herein, the parties agree as follows:
<PAGE>

1.   The primary term(s) set forth in Section 4.1 of each firm Service Agreement
     below is hereby amended to extend the primary term(s) as follows:


--------------------------------------------------------------------------------
    Original   Package     Rate     Transportation   Effective    Extended
    Contract   Number    Schedule      Demand or        Date       Primary
     Number                              MSQ                        Term
                                       (Mcf/day)
--------------------------------------------------------------------------------
    904480       1         FT           5,173        11/1/1994    10/31/2005
--------------------------------------------------------------------------------
    904481       1        FT-NN         6,764        11/1/1994    10/31/2005
--------------------------------------------------------------------------------
    S20140       1         CSS         334,977       10/30/1998   10/31/2005
--------------------------------------------------------------------------------

     Each primary term shall continue and remain in force and effect for
     successive terms of one year each after the end of each extended primary
     term for the specified Transportation Demand, unless and until cancelled
     with respect to the associated Transportation Demand by either party giving
     365 days written notice to the other party prior to the end of the extended
     primary term or 270 days written prior to the end of any yearly extension
     thereafter.

2.   Company and Shipper agree that each of the firm contracts set forth in
     Section 1 above shall be subject to a right of first refusal by Shipper at
     the expiration of the extended primary term set forth above for such
     contract(s). Notwithstanding anything in Company's FERC Gas Tariff or FERC
     Order No. 637 to the contrary, Shipper's matching bid under the right of
     first refusal provisions of Company's FERC Gas Tariff shall not have to
     exceed (a) a maximum term of five years, and (b) up to 100% of the maximum
     lawful rolled-in rate under the rate schedule that applied to the service
     as of the expiration of the extended primary term of such contract, for
     Shipper to retain its capacity. This contractual right of first refusal
     shall apply to the firm contracts of any replacement shipper that receives
     a permanent assignment, in whole or in part, of Shipper's firm contract(s)
     set forth in Section 1 above.

3.   This Amendment is subject to the conditions subsequent that: (a) the
     Settlement receives final FERC approval in accordance with Article XVI of
     the Settlement; (b) the Settlement is not otherwise terminated as provided
     in Article XVI of the Settlement; and (c) Shipper does not elect to become
     a contesting party pursuant to Article XV, paragraph 6, of the Settlement.
     If any of these conditions occur, either party may terminate this Amendment
     on ten (10) days written notice to the other party, and upon such notice,
     this Amendment shall become null and void ab initio, of no further force
     and

                                       2
















<PAGE>

     effect, and the contract provisions set forth in the contract, absent this
     amendment, shall remain in force and effect.

4.   In the event the rate moratorium under the Settlement is terminated in
     accordance with the terms of Article IV of the Settlement, Shipper shall
     have the right to reduce its Transportation Demand ("TD") under its firm
     transportation contracts during the extension period of any such contract
     and/or reduce the extension period, by giving no less than 90 days written
     notice to Company, to be given no later than one year after the effective
     date of Company's general rate change made pursuant to Article IV,
     paragraph 2, of the Settlement. Any reductions in TDs shall be pro rata
     based on TDs, among Shipper's full rate, discounted, and seasonal TDs, if
     any, and any reduction in the term of the extended contracts shall apply to
     all such contracts.

5.   The parties shall execute the attached Exhibit E to the referenced firm
     transportation contract, but it is provided that such exhibit is subject
     to the provisions of Section 3 above.

6.   This Amendment is subject to all applicable, valid laws, orders, rules and
     regulations of any governmental entity having jurisdiction over the parties
     or the subject matter hereof.

7.   This Amendment shall be binding on the parties' respective successors and
     assigns.

     WHEREFORE, the parties have executed this Amendment through their duly
authorized representatives to be effective as of the date first written above.

ATTEST:                                 SOUTHERN NATURAL GAS COMPANY

By: /s/ [ILLEGIBLE]^^                   By: Janice H. Parker
    -------------------------               -------------------------------
Title: Ass't Secretary                  Title: Director - Customer Services
       ----------------------                  ----------------------------

ATTEST:                                 ATLANTA GAS LIGHT COMPANY

By: /s/ Denise Dotson                   By: /s/ James W Scabareti
    -------------------------               -------------------------------
Title: Director, Data & Asset           Title: VP Gas Services
       ----------------------                  ----------------------------
       Management
       ----------------------

                                       3
<PAGE>

                                         Service Agreement No. 904480 and 904481
                                                               -----------------
                                            Master Service Agreement No. FSNG269
                                                         Service Type Code No. 1



                                   EXHIBIT E

                             Discount Information

Discount Transportation Rates:
During the period specified below, the Reservation Charge under this Agreement
shall be reduced by $28,700, or a Reservation Charge of $5.745225 per Dth will
be applied. All other charges and surcharges applicable to service under Rate
Schedule FT and FTNN shall be assessed at the maximum applicable rates set forth
in Company's Tariff.

Discounted Rate Effective: March 1, 2000 through July 31, 2000.


/s/ JAmes W. Scalaneto                       /s/ Janice H. Parker
------------------------------               -----------------------------------
ATLANTA GAS LIGHT COMPANY                    SOUTHERN NATURAL GAS COMPANY
        (Shipper)                                      (Company)
<PAGE>

                                   Service Agreement No.904480 and 904481
                                                        ------------------------
                                            Master Service Agreement No. FSNG269
                                                         Service Type Code No. 1



                                   EXHIBIT E

                             Discount Information


Discounted Transportation Rates:
During the period specified below, the Reservation Charge under this Agreement
shall be reduced by $21,250, or a Reservation Charge of $6.356482 per Dth will
be applied. All other charges and surcharges applicable to service under Rate
Schedule FT and FTNN shall be assessed at the maximum applicable rates set forth
in Company's Tariff.

Discounted Rate Effective: August 1, 2000 through October 31, 2005.




           /s/ James W Scabareti          /s/ Janice H. Parker
           -------------------------      ----------------------------
           ATLANTA GAS LIGHT COMPANY      SOUTHERN NATURAL GAS COMPANY
                   (Shipper)                       (Company)