EXHIBIT 5.1


                                 ANDREWS & KURTH
                          MAYOR, DAY, CALDWELL & KEETON
                                     L.L.P.

                             600 Travis, Suite 4200
                              Houston, Texas 77002



                                  June 26, 2002





Partnership Policy Committee
Northern Border Partners, L.P.
1111 South 103rd Street
Omaha, Nebraska 68124


Ladies and Gentlemen:

      We have acted as counsel to Northern Border Partners, L.P., a Delaware
limited partnership (the "Partnership"), in connection with the Partnership's
offer and sale of up to an aggregate of 2,300,000 common units (the "Common
Units") pursuant to the Partnership's Registration Statement on Form S-3
(Registration No. 333-72323) declared effective by the Securities and Exchange
Commission on March 3, 1999 (the "Registration Statement"), as supplemented by
the Prospectus Supplement, dated June 26, 2002 (the "Prospectus Supplement").

      As the basis for the opinions hereinafter expressed, we have examined such
statutes, regulations, partnership records and documents, certificates of
corporate and public officials, and other instruments as we have deemed
necessary or advisable for the purpose of this opinion. In such examinations, we
have assumed the authenticity of all documents submitted to us as originals and
the conformity with the original documents of all documents submitted to us as
copies. Without limiting the foregoing, we have examined the Underwriting
Agreement, dated June 26, 2002 (the "Underwriting Agreement"), by and between
the Partnership, Northern Border Intermediate Limited Partnership, UBS Warburg
LLC and Salomon Smith Barney Inc. (collectively, the "Underwriters").

      Based upon and subject to the foregoing and to other qualifications and
limitations set forth herein, we are of the opinion that:

      1. The Partnership is validly existing as a limited partnership under the
Delaware Revised Uniform Limited Partnership Act, as amended ("DRULPA").

      2. The Common Units have been duly authorized and, when issued and paid
for as described in the Underwriting Agreement and the Registration Statement,
as supplemented by the



Prospectus Supplement relating to the offer and sale of the Common Units, will
be duly authorized, validly issued, fully paid and non-assessable (except to the
extent such non-assessability may be affected by Sections 17-303(a) and 17-607
of the DRULPA).

      The foregoing opinions are limited to the Federal laws of the United
States of America and the DRULPA, and we are expressing no opinion as to the
effect of laws of any other jurisdiction.

      We hereby consent to the filing of this opinion as an exhibit to a Current
Report on Form 8-K of the Partnership and the reference to our firm under the
caption "Legal Matters" in the Prospectus Supplement. In giving this consent, we
do not thereby admit that we are within the category of persons whose consent is
required under Section 7 of the Securities Act of 1933, as amended, or under the
rules and regulations of the Commission relating thereto.


                                Very truly yours,



                                /s/ ANDREWS & KURTH
                                MAYOR, DAY, CALDWELL & KEETON L.L.P.