1991A NOTE

                                 PROMISSORY NOTE

                                                                   March 1, 1999

      FOR VALUE RECEIVED, MARKETSPAN CORPORATION (d/b/a KEYSPAN ENERGY), KEYSPAN
GAS EAST  CORPORATION  (d/b/a  BROOKLYN  UNION OF LONG ISLAND),  KEYSPAN  ENERGY
TRADING SERVICES LLC, KEYSPAN  GENERATION LLC, KEYSPAN  CORPORATE  SERVICES LLC,
KEYSPAN UTILITY SERVICES LLC, and KEYSPAN 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, together with their respective
successors and assigns and  predecessors,  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,  d/b/a
LIPA,  with its  principal  place of business at 333 Earle  Ovington  Boulevard,
Suite 403, Uniondale, New York 11553 (the "Obligee").

      This Promissory Note is related to the "Electric  Facilities Revenue Bonds
(Long Island Lighting Company  Project),  1991 Series A" (the "Bonds") issued by
the New York State Energy Research and Development  Authority  ("NYSERDA").  The
Obligee is required to make  payments,  among  others,  equal to the  principal,
premium, if any, and interest on the Bonds under a Participation Agreement dated
as of January 1, 1991  between  the  Obligee  and  NYSERDA  (the  "Participation
Agreement")  and a related  Corporation  Note  issued by the  Obligee to NYSERDA
under and pursuant to the Participation  Agreement (the "Corporation  Note" and,
together with the Participation Agreement, the "NYSERDA Financing Documents").

      This  Promissory  Note is a successor  to and, on and after March 1, 1999,
supercedes  in all  respects,  the original  Promissory  Note dated May 28, 1998
issued by the Obligors (or their  predecessors)  to the Obligee  relating to the
Bonds (the "Original  Promissory Note"). The Original Promissory Note was issued
pursuant to the purchase price adjustment provisions provided for by 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. (the "Merger  Agreement").  As
provided  for  by  Section  2.1(e)  of  the  Merger  Agreement,  this  successor
Promissory Note is being executed and delivered in substitution for the Original
Promissory  Note to evidence the final  agreed-upon  obligations of the Obligors
with respect to the  obligations of the Obligee  relating to the Bonds under and
as evidenced by the Participation Agreement and the Corporation Note.

      Upon the occurrence of a Material Decline in Parent's  (KeySpan  Energy'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  KeySpan  Energy,  at its sole  cost and  expense,  and (ii) may be
economically  defeased by KeySpan Energy, in each case as provided under Section
2.1(f) of the Merger Agreement.

<PAGE>

      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 and the Corporation Note. The payment obligations of the
Obligors  thereunder  shall  be  equal  to  100%  of all  such  debt  and  other
obligations  of the Obligee that remain  outstanding as of the date hereof under
the NYSERDA Financing Documents (the "Applicable Percentage").

      All provisions of the NYSERDA Financing  Documents relating to the payment
by the Obligee of principal,  interest,  premium,  if any, and any other amounts
payable thereunder,  including,  without limitation,  all provisions relating to
any  acceleration  of any such payment  obligations  for any reason,  are hereby
incorporated  herein and made a part hereof,  except that,  for purposes of such
incorporation  by reference:  (i) all references to 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 "Corporation  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) each due date under this Promissory Note for the payment by the
Obligors of amounts representing principal,  premium, if any, or interest on the
Bonds,  or payment of other amounts due under the NYSERDA  Financing  Documents,
shall be the date  which is  thirty  (30) days  prior to the due date  under the
NYSERDA  Financing  Documents  for such payment;  and (vi) the initial  Interest
Payment Date hereunder shall be 30 days prior to the next interest  payment date
on the Bonds.

      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 Obligors  hereunder,  without the prior
written  consent  of  KeySpan  Energy.  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.

      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.

 
                                        2

<PAGE>

         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.


MARKETSPAN CORPORATION
d/b/a KEYSPAN ENERGY

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer


KEYSPAN GAS EAST CORPORATION
d/b/a BROOKLYN UNION OF LONG ISLAND

By:/s/ Robert D. Ekholm
   --------------------------------------------
   Name: Robert D. Ekholm
   Title: Secretary


KEYSPAN ENERGY TRADING SERVICES LLC

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer


KEYSPAN GENERATION LLC

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer


KEYSPAN CORPORATE SERVICES LLC

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer


KEYSPAN UTILITY SERVICES LLC

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer


KEYSPAN ELECTRIC SERVICES LLC

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer


MARKETSPAN FINANCE CORPORATION

By:/s/ Robert R. Wieczorek
   --------------------------------------------
   Name: Robert R. Wieczorek
   Title: Vice President, Secretary & Treasurer