<PAGE>


                                 PROMISSORY NOTE

                                                                    May 28, 1998

         FOR VALUE RECEIVED, MARKETSPAN CORPORATION, MARKETSPAN GAS CORPORATION
(d/b/a BROOKLYN UNION), MARKETSPAN TRADING SERVICES LLC, MARKETSPAN GENERATION
LLC, MARKETSPAN CORPORATE SERVICES LLC, MARKETSPAN UTILITY SERVICES LLC, and
MARKETSPAN ELECTRIC SERVICES LLC, each a New York corporation or limited
liability company, with its principal place of business at 175 East Old Country
Road, Hicksville, New York 11801, and MARKETSPAN FINANCE CORPORATION, a Vermont
corporation (collectively, the "Obligors"), hereby jointly and severally promise
to pay, on or before the dates set forth below, the amounts set forth below, to
LONG ISLAND LIGHTING COMPANY, a New York corporation, with its principal place
of business at 333 Earle Ovington Boulevard, Suite 403, Uniondale, New York
11553 (the "Obligee").

         This Promissory Note is one of a series of Promissory Notes issued
pursuant to the purchase price adjustment provisions of Section 2.1 of the
Agreement and Plan of Merger dated as of June 26, 1997 by and among MarketSpan
Corporation (formerly known as BL Holding Corp.), the Obligee, Long Island Power
Authority and LIPA Acquisition Corp., as amended and/or supplemented through the
date hereof (as so amended and/or supplemented, the "Merger Agreement"), and is
subject to the terms and provisions thereof relating hereto. Upon the occurrence
of a Material Decline in Parent's Credit Standing (as such term is defined in
Section 2.1(f)(ii) of the Merger Agreement) this Promissory Note (i) shall be
secured by a letter of credit provided by MarketSpan Corporation, at its sole
cost and expense, and (ii) may be economically defeased by MarketSpan
Corporation, in each case as provided under Section 2.1(f) of the Merger
Agreement.

         The terms and conditions of this Promissory Note shall be determined by
reference to the debt and other obligations of the Obligee under the
Participation Agreement dated as of October 1, 1994, between New York State
Energy Research and Development Authority ("NYSERDA") and the Obligee (the
"Participation Agreement"), relating to the Electric Facilities Revenue Bonds
(Long Island Lighting Company Project), 1994 Series A issued by NYSERDA, and the
related Company Note issued by the Obligee thereunder (collectively, the
"NYSERDA Financing Documents"). The payment obligations of the Obligors
hereunder shall equal 8.6000% of all such debt and other obligations of the
Obligee under the NYSERDA Financing Documents, or such other percentage as may
be finally determined in accordance with Section 2.1(e) of the Merger
Agreement (the "Applicable Percentage"). In the event that the Applicable
Percentage changes, as provided above, following the date hereof, the
appropriate party shall pay to the other, within five (5) Business Days (as
defined in the NYSERDA Financing Documents) following such change, all such
amounts as shall be due upon the application of the Applicable Percentage
retroactively to the date hereof.


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          All provisions of the NYSERDA Financing Documents relating to the
payment by the Obligee of principal, interest, premium and any other amounts
payable thereunder, including, without limitation, all provisions relating to
(A) any acceleration of any such payment obligations for any reason and (B) any
fees or other expenses relating to letters of credit, are hereby incorporated
herein and made a part hereof, except that, for purposes of such incorporation
by reference, (i) all references to the "Company" shall be deemed to be
references to the Obligors jointly and severally), (ii) all references to the
"Authority" or the "Trustee" shall be deemed to be references to the Obligee,
(iii) all references to the "Company Note" shall be deemed to be references to
this Promissory Note, (iv) all references to amounts payable shall be deemed to
be references to such amount multiplied by the Applicable Percentage, (v) all
dates specified therein for payments shall be deemed to be the respective dates
thirty (30) days prior thereto, (vi) the principal amount hereof shall bear
interest commencing on the date thirty (30) days prior to the immediately
preceding Interest Payment Date under the NYSERDA Financing Documents, (vii) the
initial Interest Payment Date hereunder shall be May 28, 1998 and (viii) the
obligation of the Obligors to make any payment of principal of premium, if any,
and interest on, this Promissory Note shall NOT be deemed satisfied and
discharged to the extent of any payment made by the Bank under the Letter of
Credit. In addition to the events and circumstances included in the definition
of the term "events of default" incorporated herein from the Participation
Agreement, such term, as used in this Promissory Note, shall include the
occurrence of any event of default under the Participation Agreement. In the
event that the amount of interest paid by the Obligors on any Interest Payment
Date hereunder differs from the amount of interest otherwise payable by the
Obligee on the immediately succeeding Interest Payment Date (the "Obligee 
Payment Date") under the NYSERDA Financing Documents (whether or not any of the
obligations under the NYSERDA Financing Documents remain outstanding) multiplied
by the Applicable Percentage (the "NYSERDA Interest"), the amount of such
difference shall be paid by the appropriate party to the other, within five (5)
Business Days following the Obligee Payment Date, so that the total interest
paid hereunder shall equal the total NYSERDA Interest.

          Each of the Obligors hereby waives presentment for payment, demands,
notice of dishonor and protest of this Promissory Note and any right to assert
setoff of any of its obligations hereunder against any amounts owing by the
Obligee thereto. The Obligors further agree that none of the terms or provisions
of this Promissory Note may be waived, altered, modified or amended except as
the Obligee may consent in a writing duly signed for and on its behalf. The
Obligee agrees that it shall not agree to any alteration, modification or
amendment of any of the NYSERDA Financing Documents in any way that would
increase the payment obligations of the Obligees hereunder, without the prior
written consent of MarketSpan Corporation. No failure or delay on the part of
the Obligee in exercising any of its rights, powers or privileges hereunder
shall operate as a waiver thereof, nor shall a single or partial exercise
thereof preclude any other or further exercise of any right, power or privilege.
The remedies provided herein are cumulative and not exclusive of any remedies
provided by law.

          The Obligors agree to pay on demand any and all reasonable
out-of-pocket costs and expenses (including reasonable fees and expenses of
counsel) incurred by the Obligee and its successor and assigns in enforcing this
Promissory Note.


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         This Promissory Note is binding upon the Obligors and their successors
and assigns and is for the benefit of the Obligee and its successors and
assigns. The Obligors may not, without the prior written consent of the Obligee,
assign or otherwise transfer their obligations under this Promissory Note.

         This Promissory Note has been delivered in the State of New York and
shall be construed and enforced in accordance with the internal and substantive
laws of such State without giving effect to the choice of law rules.


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                                         MARKETSPAN CORPORATION

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President


                                         MARKETSPAN GAS CORPORATION

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President


                                         MARKETSPAN TRADING SERVICES 
                                         LLC

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President


                                         MARKETSPAN GENERATION LLC

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President


                                       4


<PAGE>


                                         MARKETSPAN CORPORATE SERVICES 
                                         LLC

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President


                                         MARKETSPAN UTILITY SERVICES 
                                         LLC

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President
                                          

                                         MARKETSPAN ELECTRIC SERVICES
                                         LLC

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President

                                         MARKETSPAN FINANCE 
                                         CORPORATION

                                         By: /s/ Kathleen A. Marion
                                           ------------------------
                                           Name: Kathleen A. Marion
                                           Title: Secretary and Vice President

                                       5