<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 November 1, 1993, 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), 1993 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 32.1000% 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" or the
"Corporation" 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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<PAGE>

         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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<PAGE>

                                        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


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<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


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