<PAGE>

                UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                                    FORM 10-Q

[ X ]  QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
       EXCHANGE ACT OF 1934
                                       or

[   ]  TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
       EXCHANGE ACT OF 1934

FOR THE QUARTERLY PERIOD ENDED SEPTEMBER 30, 1999


                        Commission File Number: 000-26091
                                TC PIPELINES, LP
             (Exact name of registrant as specified in its charter)


                DELAWARE                                  52-2135448
     (State or other jurisdiction of                   (I.R.S. Employer
      incorporation or organization)                  Identification Number)


       FOUR GREENSPOINT PLAZA
       16945 NORTHCHASE DRIVE
           HOUSTON, TEXAS                                      77060
(Address of principal executive offices)                    (Zip code)



                                 (281) 873-7774
               Registrant's telephone number, including area code


     Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such report(s), and (2) has been subject to
such filing requirements for the past 90 days.
Yes [X]             No [ ]

     The registrant had 14,690,694 common units outstanding as of
November 10, 1999.

<PAGE>

                                TC PIPELINES, LP

                                TABLE OF CONTENTS
<TABLE>
<CAPTION>
                                                                                             Page No.
                                                                                             --------
<S>                                                                                          <C>
PART I.  FINANCIAL INFORMATION

     ITEM 1.  Financial Statements

          Income - Three months ended September 30, 1999 and
              Period May 28 to September 30, 1999                                                3
          Balance Sheet - September 30, 1999 and May 28, 1999                                    3
          Cash Flow - Period May 28 to September 30, 1999                                        4
          Notes to Condensed Financial Statements                                                5

     ITEM 2.  Management's Discussion and Analysis of Financial Condition and
              Results of Operations

          Results of Operations of TC PipeLines, LP                                              8
          Liquidity and Capital Resources of TC PipeLines, LP                                    8
          Results of Operations of Northern Border Pipeline Company                             10
          Liquidity and Capital Resources of Northern Border Pipeline Company                   13

     ITEM 3.  Quantitative and Qualitative Disclosures About Market Risk                        18

PART II.  OTHER INFORMATION

     ITEM 6.  Exhibits and Reports on Form 8-K                                                  19
</TABLE>


                                      2
<PAGE>

                        PART I. FINANCIAL INFORMATION

                         ITEM 1. FINANCIAL STATEMENTS

                               TC PIPELINES, LP

                                   INCOME

<TABLE>
<CAPTION>
(unaudited)                                                    THREE MONTHS ENDED          May 28 (1) -
(thousands of dollars except per unit amounts)                 SEPTEMBER 30, 1999      September 30, 1999
---------------------------------------------------------------------------------------------------------
<S>                                                            <C>                     <C>
EQUITY INCOME FROM INVESTMENT IN
  NORTHERN BORDER PIPELINE COMPANY (NOTE 3)                                 8,738                 11,868
GENERAL AND ADMINISTRATIVE EXPENSES                                           239                    383
                                                               -------------------    -------------------
NET INCOME                                                                  8,499                 11,485
                                                               -------------------    -------------------
                                                               -------------------    -------------------

NET INCOME ALLOCATION
General partner                                                               170                    230
Common units                                                                6,992                  9,448
Subordinated units                                                          1,337                  1,807
                                                               -------------------    -------------------
                                                                            8,499                 11,485
                                                               -------------------    -------------------
                                                               -------------------    -------------------


NET INCOME PER UNIT (NOTE 4)                                                $0.48                  $0.64
                                                               -------------------    -------------------
                                                               -------------------    -------------------
UNITS OUTSTANDING (THOUSANDS)                                              17,500                 17,500
                                                               -------------------    -------------------
                                                               -------------------    -------------------
</TABLE>



                                     BALANCE SHEET

<TABLE>
<CAPTION>
(unaudited)
(thousands of dollars)                                         SEPTEMBER 30, 1999       May 28, 1999 (1)
---------------------------------------------------------------------------------------------------------
<S>                                                            <C>                    <C>
ASSETS
Cash                                                                          544                      -
Investment in Northern Border Pipeline Company (Note 3)                   250,169                241,651
                                                               -------------------    -------------------
                                                               -------------------    -------------------
                                                                          250,713                241,651
                                                               -------------------    -------------------
                                                               -------------------    -------------------

LIABILITIES AND PARTNERS' CAPITAL
Current Liabilities
  Due to affiliate                                                            300                      -
  Accounts payable                                                            279                      -
                                                               -------------------    -------------------
                                                               -------------------    -------------------
                                                                              579                      -
                                                               -------------------    -------------------
Partners' Capital
  General partner                                                           5,003                  4,833
  Common units                                                            207,994                193,515
  Subordinated units                                                       37,137                 43,303
                                                               -------------------    -------------------
                                                               -------------------    -------------------
                                                                          250,134                241,651
                                                               -------------------    -------------------
                                                               -------------------    -------------------
                                                                          250,713                241,651
                                                               -------------------    -------------------
                                                               -------------------    -------------------
</TABLE>


See accompanying Notes to Condensed Financial Statements.

(1)  Commencement of operations


                                       3

<PAGE>

                  PART I. FINANCIAL INFORMATION (CONTINUED)

                   ITEM 1. FINANCIAL STATEMENTS (CONTINUED)

                               TC PIPELINES, LP

                                    CASH FLOW
<TABLE>
<CAPTION>
(unaudited)                                                                             MAY 28 (1) -
(thousands of dollars)                                                               SEPTEMBER 30, 1999
--------------------------------------------------------------------------------------------------------
<S>                                                                                  <C>
CASH GENERATED FROM OPERATIONS
Net income                                                                                       11,485
Add/(Deduct):
Equity income in excess of distributions received                                                (8,518)
Decrease in operating working capital                                                               279
                                                                                     -------------------
                                                                                                  3,246
                                                                                     -------------------

FINANCING ACTIVITIES
Distributions paid                                                                               (3,002)
Due to affiliate                                                                                    300
Common units issued                                                                               7,501
Subordinated units redeemed                                                                      (7,501)
                                                                                     -------------------
                                                                                                 (2,702)
                                                                                     -------------------

INCREASE IN CASH                                                                                    544

CASH, BEGINNING OF PERIOD                                                                             -
                                                                                     -------------------
CASH, END OF PERIOD                                                                                 544
                                                                                     -------------------
                                                                                     -------------------
</TABLE>


See accompanying Notes to Condensed Financial Statements.

(1)  Commencement of operations


                                        4
<PAGE>


                  PART I. FINANCIAL INFORMATION (CONTINUED)

                   ITEM 1. FINANCIAL STATEMENTS (CONTINUED)

                               TC PIPELINES, LP

                   NOTES TO CONDENSED FINANCIAL STATEMENTS
   For the period May 28 (commencement of operations) to September 30, 1999
                                 (unaudited)

NOTE 1    BASIS OF PRESENTATION

TC PipeLines, LP, a Delaware limited partnership, and its subsidiary limited
partnership, TC PipeLines Intermediate Limited Partnership, a Delaware
limited partnership, are collectively referred to herein as TC PipeLines or
the Partnership.

     The financial statements have been prepared by management in accordance
with United States generally accepted accounting principles. Amounts are
stated in United States dollars.

     Since a determination of many assets, liabilities, revenues and expenses
is dependent upon future events, the preparation of these financial statements
requires the use of estimates and assumptions which have been made using careful
judgment. In the opinion of management, these financial statements have been
properly prepared within reasonable limits of materiality and include all
adjustments (consisting primarily of normal recurring accruals) necessary to
present fairly the results of operations, financial position and cash flows of
the Partnership as at May 28, 1999 and September 30, 1999, and for the three
months ended September 30, 1999 and for the period May 28 to September 30, 1999.

     The results of operations for the three months ended September 30, 1999
and the period May 28 to September 30, 1999, are not necessarily indicative
of the results that may be expected for a full fiscal year.

NOTE 2    FORMATION OF PARTNERSHIP

TC PipeLines was formed on December 16, 1998. The Partnership commenced
operations on May 28, 1999 when it issued 14,300,000 common units (11,500,000
to the public and 2,800,000 to an affiliate of the general partner) for net
proceeds of $274.6 million, after deducting underwriters' fees of $15.0
million. These proceeds, along with 3,200,000 subordinated units, a 2 percent
general partner interest and incentive distribution rights, were issued to
TransCanada Border PipeLine Ltd. and TransCan Northern Ltd. (collectively,
the predecessor companies), affiliates of the general partner, to acquire the
predecessor companies' 30 percent general partner interest in Northern Border
Pipeline Company.

     On June 25, 1999, the underwriters exercised a portion of their
over-allotment option under the terms of the underwriting agreement and
purchased 390,694 additional common units for proceeds of $7.5 million. The
Partnership used those proceeds to redeem 390,694 subordinated units.

     The common units and the subordinated units represent limited partner
interests in the Partnership. During the period which subordinated units are
outstanding (the subordination period), to the extent there is sufficient
available cash, the holders of common units are entitled to receive a minimum
quarterly distribution (MQD), plus any arrearages on the common units, before
any distribution is made to the holders of subordinated units. The holders of
subordinated units will have the right to receive the MQD only after the
common units have received the MQD plus any arrearages in payment of the MQD.
The subordinated units are not entitled to arrearages. Upon expiration of the
subordination period, which will generally not occur before June 30, 2004,
the subordinated units will convert into common units on a one-for-one basis
and will then participate pro rata with the other common units in
distributions of available cash.

     The holder of the general partner interest is entitled to receive 2
percent of total cash distributions until the MQD has been achieved, at which
time it will have the right to receive incentive distributions. Incentive
distribution rights represent the right to receive an increasing percentage
of quarterly distributions of available cash after the MQD has been achieved.

NOTE 3    INVESTMENT IN NORTHERN BORDER PIPELINE COMPANY

The Partnership owns a 30 percent general partner interest in Northern Border
Pipeline Company (Northern Border), a partnership which owns a natural gas
pipeline extending from the Montana-Saskatchewan border near Port of Morgan,
Montana, to a terminus near Manhattan, Illinois. Northern Border is subject
to regulation by the Federal Energy Regulatory Commission. Northern Border's
accounting policies conform to U.S. generally accepted accounting principles,
as applied in the case of regulated entities.


                                       5

<PAGE>

                  PART I. FINANCIAL INFORMATION (CONTINUED)

                   ITEM 1. FINANCIAL STATEMENTS (CONTINUED)

                               TC PIPELINES, LP

     The Partnership uses the equity method of accounting for its investment
in Northern Border, over which it is able to exercise significant influence.
The Partnership's investment balance as at May 28, 1999 represents the
combined carrying values of the investment in Northern Border as reflected in
the accounts of the predecessor companies at the same date. TC PipeLines'
equity income for the three months ended September 30, 1999 and the period
May 28 to September 30, 1999 represents 30 percent of the net income of
Northern Border for the same periods.

     The following sets out summarized financial information for Northern
Border for the three months ended September 30, 1999, the period May 28 to
September 30, 1999 and as at September 30, 1999. TC PipeLines has held its
general partner interest since May 28, 1999.

<TABLE>
<CAPTION>
(UNAUDITED)                                                       THREE MONTHS ENDED           MAY 28 -
(MILLIONS OF DOLLARS)                                             SEPTEMBER 30, 1999      SEPTEMBER 30, 1999
-------------------------------------------------------------------------------------------------------------
<S>                                                               <C>                    <C>
NORTHERN BORDER INCOME STATEMENT
Revenues                                                                       73.9                   101.5
Costs and expenses                                                            (16.8)                  (23.1)
Depreciation                                                                  (13.1)                  (18.0)
Financial charges and other                                                   (14.9)                  (20.9)
                                                                   ------------------    --------------------
                                                                   ------------------    --------------------
Net income                                                                     29.1                    39.5
                                                                   ------------------    --------------------
                                                                   ------------------    --------------------
</TABLE>

<TABLE>
<CAPTION>
(UNAUDITED)
(MILLIONS OF DOLLARS)                                             SEPTEMBER 30, 1999
-------------------------------------------------------------------------------------
<S>                                                               <C>
NORTHERN BORDER BALANCE SHEET
Cash and short-term investments                                                 20.6
Other current assets                                                            31.0
Plant, property and equipment, net                                           1,739.1
Other assets                                                                    14.5
Current liabilities                                                           (126.1)
Deferred amounts                                                               (10.4)
Long-term debt                                                                (834.8)
                                                                  -------------------
Partners' capital                                                              833.9
                                                                  -------------------
                                                                  -------------------
</TABLE>

NOTE 4    NET INCOME PER UNIT

Net income per unit is computed by dividing net income, after deduction of
the general partner's allocation, by the number of common and subordinated
units outstanding.


                                       6
<PAGE>

                  PART I. FINANCIAL INFORMATION (CONTINUED)

                   ITEM 1. FINANCIAL STATEMENTS (CONCLUDED)

                               TC PIPELINES, LP


NOTE 5    CREDIT FACILITY

On May 28, 1999, the Partnership entered into a $40 million unsecured
two-year revolving credit facility with TransCanada PipeLine USA Ltd., an
affiliate of the general partner. At September 30, 1999, the Partnership had
no amount outstanding under this credit facility.


                                       7
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
                  FINANCIAL CONDITION AND RESULTS OF OPERATIONS

                                TC PIPELINES, LP


RESULTS OF OPERATIONS OF TC PIPELINES, LP

     TC PipeLines, LP (the Partnership) was formed by TransCanada PipeLines
Limited (TransCanada) to acquire, own and participate in the management of
United States based pipeline assets. On May 28, 1999, the Partnership issued
14,300,000 common units (11,500,000 to the public and 2,800,000 to an
affiliate of the general partner) for net proceeds of $274.6 million. The
common units are quoted on the Nasdaq National Market and trade under the
symbol "TCLPZ". The Partnership used the net proceeds from this offering,
along with 3,200,000 subordinated units, an aggregate 2 percent general
partner interest and incentive distribution rights, to acquire the collective
30 percent general partner interest in Northern Border Pipeline Company
(Northern Border) previously held by TransCanada Border PipeLine Ltd. and
TransCan Northern Ltd. (collectively, the predecessor companies), affiliates
of the general partner, TC PipeLines GP, Inc.

     Subsequent to the initial public offering, the underwriters exercised a
portion of their over-allotment option and purchased 390,694 additional common
units for proceeds of $7.5 million. The Partnership used these proceeds to
redeem an equal number of subordinated units.

     Currently, the only material asset of the Partnership is the 30 percent
general partner interest in Northern Border.

     TC PipeLines, LP accounts for its interest in Northern Border using the
equity method of accounting. The Partnership's initial investment in Northern
Border was recorded at $241.7 million, the combined carrying values of the
investment in Northern Border as reflected in the accounts of the predecessor
companies as at May 28, 1999. This amount equated to 30 percent of Northern
Border's partners' capital as at May 28, 1999.

     Since the general partner interest in Northern Border is currently the
Partnership's only source of income, the Partnership's results of operations
are influenced by and reflect the same factors that influence the financial
results of Northern Border.

THIRD QUARTER 1999

     For the three months ended September 30, 1999, TC PipeLines recorded
$8.7 million of equity income from Northern Border and incurred general and
administrative expenses of $0.2 million, resulting in net income of $8.5
million for the third quarter of 1999.

PERIOD MAY 28 TO SEPTEMBER 30, 1999

     TC PipeLines recorded $11.9 million of equity income from Northern
Border for the period May 28 to September 30, 1999 and incurred general and
administrative expenses of $0.4 million, resulting in net income of $11.5 for
the same period.

LIQUIDITY AND CAPITAL RESOURCES OF TC PIPELINES, LP

CASH DISTRIBUTION POLICY OF TC PIPELINES, LP

     During the period when subordinated units are outstanding (the
subordination period), which generally cannot end before June 30, 2004, the
Partnership will make distributions of available cash as defined in the
partnership agreement in the following manner:

     -  First, 98 percent to the common units, pro rata, and 2 percent to the
        general partner, until there has been distributed for each outstanding
        common unit an amount equal to the minimum quarterly distribution for
        that quarter;

     -  Second, 98 percent to the common units, pro rata, and 2 percent to the
        general partner, until there has been distributed for each outstanding
        common unit an amount equal to any arrearages in payment of the minimum
        quarterly distribution on the common units for that quarter and for any
        prior quarters during the subordination period;


                                      8
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


     -  Third, 98 percent to the subordinated units, pro rata, and 2 percent to
        the general partner, until there has been distributed for each
        outstanding subordinated unit an amount equal to the minimum quarterly
        distribution for that quarter; and

     -  Thereafter, in a manner whereby the general partner has rights
        (referred to as Incentive Distribution Rights) to receive increasing
        percentages of quarterly distributions.

     The initial distribution of $0.1681 per unit, paid on August 12, 1999,
reflected the minimum quarterly distribution as prorated for the shortened
period from the May 28, 1999 commencement of operations to June 30, 1999.

PERIOD MAY 28 TO SEPTEMBER 30,1999

     On August 12, 1999, TC PipeLines paid a cash distribution of $0.1681 per
unit for the period May 28 to June 30, 1999, to unitholders of record as of
July 30, 1999. This cash distribution of $3.0 million was paid out in the
following manner: $2.5 million to common unitholders, $0.5 million to the
general partner as holder of the subordinated units, and the balance to the
general partner in respect of its 2 percent general partner interest.

     The Partnership funded this cash distribution with its share of Northern
Border's second quarter cash distribution, which was received on August 3,
1999.

     On October 19, 1999, the board of directors of the general partner
declared a cash distribution of $0.45 per unit for the three months ended
September 30, 1999, which is payable on November 12, 1999, to unitholders of
record as of October 29, 1999. This is the Partnership's first distribution
for a full quarter. This will amount to a cash distribution totaling $8.0
million, which will be paid out in the following manner: $6.6 million to
common unitholders, $1.2 million to the general partner as holder of the
subordinated units, and $0.2 million to the general partner in respect of its
2 percent general partner interest.

     The Partnership will fund this cash distribution with its share of
Northern Border's third quarter cash distribution, which was received on
November 2, 1999.

NORTHERN BORDER CASH DISTRIBUTION POLICY

     In December 1998, Northern Border changed its policy regarding
distributions to its partners in two respects, with the unanimous consent of
the members of its management committee. The timing of distributions was
changed from the last business day of the quarter to the second business day
of the second month following the end of the quarter. The computation period
for distributions was also changed from quarterly with one month in arrears
to quarterly.

     In accordance with Northern Border's cash distribution policy, a
distribution was made to its general partners on August 3, 1999, for the
second quarter ending June 30, 1999. As stated in the amended general
partnership agreement for Northern Border, the predecessor companies received
their proportionate share of this cash distribution for the period April 1 to
May 27, 1999. TC PipeLines, LP received $3.3 million, representing 30 percent
of Northern Border's cash distribution for the period May 28 to June 30, 1999.

     In accordance with Northern Border's cash distribution policy, a
distribution for the third quarter ending September 30, 1999 was paid on
November 2, 1999. TC PipeLines, LP received $8.8 million, representing 30
percent of that cash distribution.

CREDIT FACILITY AND SHORT-TERM BORROWINGS

     On May 28, 1999, the Partnership entered into a $40 million unsecured
two-year revolving credit facility with TransCanada PipeLine USA Ltd., an
affiliate of the general partner. The credit facility bears interest at a
London Interbank Offered Rate plus 1.25 percent. The purpose of the revolving
credit facility is to provide borrowings to

                                      9
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


fund capital expenditures, to fund capital contributions to Northern Border
and for working capital and other general business purposes, including
funding cash distributions to partners, if necessary. At September 30, 1999,
the Partnership had no amount outstanding under this credit facility.

     On June 28, 1999, the Partnership received a short-term, non-interest
bearing working capital advance in the amount of $0.3 million from its
general partner. The Partnership intends to repay this advance within twelve
months.

CAPITAL REQUIREMENTS

     The Partnership expects that Northern Border will request capital
contributions in aggregate of approximately $9.0 million from TC PipeLines, LP
in the year 2000. Management expects to fund these capital contributions from
cash reserves and drawings under the revolving credit facility.

RESULTS OF OPERATIONS OF NORTHERN BORDER PIPELINE COMPANY

     The following sets out summarized financial information for Northern
Border for the three and nine months ended September 30, 1999, and as at
September 30, 1999. TC PipeLines, LP has held its 30 percent general partner
interest since May 28, 1999.

<TABLE>
<CAPTION>

(UNAUDITED)                                          THREE MONTHS ENDED        NINE MONTHS ENDED
(MILLIONS OF DOLLARS)                                SEPTEMBER 30, 1999        SEPTEMBER 30, 1999
-------------------------------------------------------------------------     --------------------
<S>                                                  <C>                      <C>
NORTHERN BORDER INCOME STATEMENT
Revenues                                                            73.9                    220.6
Costs and expenses                                                 (16.8)                   (49.7)
Depreciation                                                       (13.1)                   (38.8)
Financial charges and other                                        (14.9)                   (43.7)
                                                     --------------------      -------------------

Net income                                                           29.1                    88.4
                                                     --------------------      -------------------
                                                     --------------------      -------------------
</TABLE>


<TABLE>
<CAPTION>

(UNAUDITED)
(MILLIONS OF DOLLARS)                                SEPTEMBER 30, 1999
-------------------------------------------------------------------------
<S>                                                  <C>
NORTHERN BORDER BALANCE SHEET
Cash and short-term investments                                     20.6
Other current assets                                                31.0
Plant, property and equipment, net                               1,739.1
Other assets                                                        14.5
Current liabilities                                               (126.1)
Deferred amounts                                                   (10.4)
Long-term debt                                                    (834.8)
                                                     --------------------

Partners' capital                                                  833.9
                                                     --------------------
</TABLE>

     Northern Border's revenue is derived from agreements with various
shippers for the transportation of natural gas. It transports gas under a
Federal Energy Regulatory Commission ("FERC") regulated tariff that provides
an

                                      10
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


opportunity to recover operations and maintenance costs of the pipeline,
taxes other than income taxes, interest, depreciation and amortization, an
allowance for income taxes and a regulated return on equity. Northern Border
is generally allowed an opportunity to collect from its shippers a return on
regulated rate base as well as recover that rate base through depreciation
and amortization. The return amount Northern Border may collect from its
shippers declines as the rate base is recovered. Billings for the firm
transportation agreements are based on contracted volumes to determine the
allocable share of the cost of service and are not dependent upon the
percentage of available capacity actually used.

     In December 1998, Northern Border completed an expansion and extension
of its pipeline system (The Chicago Project). As a result of placing the
facilities for The Chicago Project into service, Northern Border has added
approximately $895 million to its gas plant in service through September 30,
1999 ($840 million through December 31, 1998).

THIRD QUARTER 1999 COMPARED WITH THIRD QUARTER 1998

     Operating revenues increased $24.8 million for the third quarter of
1999, as compared to the same period in 1998, due primarily to additional
revenues from the operation of The Chicago Project facilities. New firm
transportation agreements with 27 shippers provided for additional receipt
capacity of 700 million cubic feet per day, a 42% increase. The Chicago
Project increased Northern Border's rate base, which increased Northern
Border's return for the third quarter of 1999. Also reflected in the increase
in 1999 revenues are recoveries of increased pipeline operating expenses due
to the new facilities.

     Costs and expenses consist of operations and maintenance expense, taxes
other than income and the regulatory credit account.

     Operations and maintenance expense increased $2.6 million for the third
quarter of 1999, from the comparable period in 1998, due primarily to
operations and maintenance expenses for The Chicago Project facilities
recorded in 1999 and increased administrative expenses for Northern Border.

     Taxes other than income increased $1.7 million for the third quarter of
1999, as compared to the same period in 1998, due primarily to ad valorem
taxes attributable to the facilities placed into service for The Chicago
Project.

     For the third quarter of 1998, Northern Border recorded a regulatory
credit of $2.5 million. During the construction of The Chicago Project,
Northern Border placed certain new facilities into service in advance of the
December 1998 in service date to maintain gas flow at firm contracted
capacity while existing facilities were being modified. The regulatory credit
deferred the cost of service of these new facilities. Northern Border is
allowed to recover from its shippers the regulatory asset, that resulted from
the cost of service deferral, over a ten-year period commencing with the in
service date of The Chicago Project.

     Depreciation and amortization expense increased $2.9 million for the
third quarter of 1999, as compared to the same period in 1998, due primarily
to Northern Border placing the facilities for The Chicago Project into
service. The impact of the additional facilities on depreciation and
amortization expense was partially offset by a decrease in the depreciation
rate applied to transmission plant from 2.5% to 2.0%. Northern Border agreed
to reduce the depreciation rate at the time The Chicago Project was placed
into service, as part of a previous rate case settlement.

     Financial charges and other consists of interest expense, net and other
income.

     Interest expense, net increased $9.5 million for the third quarter of
1999, as compared to the same period in 1998, due to an increase in interest
expense of $3.7 million and a decrease in interest expense capitalized of
$5.8 million. Interest expense increased due primarily to an increase in
average debt outstanding, reflecting amounts borrowed to finance a portion of
the capital expenditures for The Chicago Project. The impact of the increased
borrowings on interest expense was partially offset by a decrease in Northern
Border's average interest rates between 1998 and 1999. The decrease in
interest expense capitalized is due to the completion of construction of The
Chicago Project in December 1998.


                                      11
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


     Other income decreased $3.5 million for the third quarter of 1999, as
compared to the same period in 1998, primarily due to a decrease in the
allowance for equity funds used during construction. The decrease in the
allowance for equity funds used during construction is due to the completion
of construction of The Chicago Project in December 1998.

NINE MONTHS ENDED SEPTEMBER 30, 1999 COMPARED WITH NINE MONTHS ENDED
SEPTEMBER 30, 1998

     Operating revenues increased $75.1 million for the nine months ended
September 30, 1999, as compared to the same period in 1998, due primarily to
additional revenues from the operation of The Chicago Project facilities. The
Chicago Project increased Northern Border's rate base, which increased
Northern Border's return for the nine months ended September 30, 1999. Also
reflected in the increase in 1999 revenues are recoveries of increased
pipeline operating expenses due to the new facilities.

     Costs and expenses consist of operations and maintenance expense, taxes
other than income and the regulatory credit account.

     Operations and maintenance expense increased $6.2 million for the nine
months ended September 30, 1999, from the comparable period in 1998, due
primarily to operations and maintenance expenses for The Chicago Project
facilities recorded in 1999 and increased administrative expenses for
Northern Border.

     Taxes other than income increased $4.6 million for the nine months ended
September 30, 1999, as compared to the same period in 1998, due primarily to
ad valorem taxes attributable to the facilities placed into service for The
Chicago Project.

     For the nine months ended September 30, 1998, Northern Border recorded a
regulatory credit of $4.7 million. During the construction of The Chicago
Project, Northern Border placed certain new facilities into service in
advance of the December 1998 in service date to maintain gas flow at firm
contracted capacity while existing facilities were being modified. The
regulatory credit deferred the cost of service of these new facilities.
Northern Border is allowed to recover from its shippers the regulatory asset,
that resulted from the cost of service deferral, over a ten-year period
commencing with the in service date of The Chicago Project.

     Depreciation and amortization expense increased $8.7 million for the
nine months ended September 30, 1999, as compared to the same period in 1998,
due primarily to Northern Border placing the facilities for The Chicago
Project into service. The impact of the additional facilities on depreciation
and amortization expense was partially offset by a decrease in the
depreciation rate applied to transmission plant from 2.5% to 2.0%. Northern
Border agreed to reduce the depreciation rate at the time The Chicago Project
was placed into service, as part of a previous rate case settlement.

     Financial charges and other consists of interest expense, net and other
income.

     Interest expense, net increased $25.9 million for the nine months ended
September 30, 1999, as compared to the same period in 1998, due to an
increase in interest expense of $13.9 million and a decrease in interest
expense capitalized of $12.0 million. Interest expense increased due,
primarily, to an increase in average debt outstanding, reflecting amounts
borrowed to finance a portion of the capital expenditures for The Chicago
Project. The impact of the increased borrowings on interest expense was
partially offset by a decrease in Northern Border's average interest rates
between 1998 and 1999. The decrease in interest expense capitalized is due to
the completion of construction of The Chicago Project in December 1998.

     Other income decreased $8.5 million for the nine months ended September
30, 1999, as compared to the same period in 1998, primarily due to a decrease
in the allowance for equity funds used during construction. The decrease in
the allowance for equity funds used during construction is due to the
completion of construction of The Chicago Project in December 1998.


                                      12
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


LIQUIDITY AND CAPITAL RESOURCES OF NORTHERN BORDER PIPELINE COMPANY

GENERAL

     In June 1997, Northern Border entered into a credit agreement (Pipeline
Credit Agreement) with certain financial institutions to borrow up to an
aggregate principal amount of $750 million. The Pipeline Credit Agreement is
comprised of a $200 million five-year revolving credit facility to be used
for the retirement of Northern Border's previously outstanding bank loan
agreements and for general business purposes, and a $550 million three-year
revolving credit facility to be used for the construction of The Chicago
Project. Effective March 1999, in accordance with the provisions of the
Pipeline Credit Agreement, Northern Border converted the three-year revolving
credit facility to a term loan maturing in June 2002. At September 30, 1999,
$439.0 million was outstanding on the term loan and there was not any
outstanding amount on the five-year revolving credit facility.

     In August 1999, Northern Border completed a private offering of $200
million of 7.75% Senior Notes due 2009 (Senior Notes). In October 1999,
Northern Border filed a registration statement relating to the Senior Notes,
which has not yet become effective with the Securities and Exchange
Commission. Also in August 1999, Northern Border received approximately $12.9
million from the termination of interest rate forward agreements, which is
included in long-term debt on the balance sheet and amortized against
interest expense over the life of the Senior Notes. The interest rate forward
agreements, which had an aggregate notional amount of $150 million, had been
executed in September 1998 to hedge the interest rate on a planned issuance
of fixed rate debt in 1999. The proceeds from the private offering, net of
associated debt discounts and issuance costs, and the termination of the
interest rate forward agreements were used to reduce existing indebtedness
under the Pipeline Credit Agreement.

     Short-term liquidity needs will be met by internal sources and through
the lines of credit discussed above. Long-term capital needs may be met
through the ability to issue long-term indebtedness.

CASH FLOWS FROM OPERATING ACTIVITIES

     Cash flows provided by operating activities increased $49.9 million to
$133.3 million for the nine months ended September 30, 1999, as compared to
the same period in 1998, primarily attributed to The Chicago Project
facilities placed into service in late December 1998.

CASH FLOWS FROM INVESTING ACTIVITIES

     Capital expenditures of $89.6 million for the nine months ended
September 30, 1999 include $78.3 million for The Chicago Project. The
remaining capital expenditures for 1999 are primarily related to renewals and
replacements of Northern Border's existing facilities. For the comparable
period in 1998, capital expenditures were $484.2 million, which included
$474.4 million for The Chicago Project.

     Total capital expenditures for 1999 are estimated to be $111 million
including $89 million for The Chicago Project. Approximately $37 million of
the capital expenditures for The Chicago Project are for construction
completed in 1998. The remaining $22 million of 1999 capital expenditures is
planned primarily for renewals and replacements of the existing facilities.
Northern Border anticipates funding its 1999 capital expenditures primarily
by borrowing on the Pipeline Credit Agreement and using internal sources.

CASH FLOWS FROM FINANCING ACTIVITIES

     Cash flows used in financing activities were $60.6 million for the nine
months ended September 30, 1999, as compared to cash flows provided by
financing activities of $400.8 million for the nine months ended September
30, 1998. During the nine months ended September 30, 1998, Northern Border's
general partners contributed $197.0 million to finance a portion of the
capital expenditures for The Chicago Project. Distributions paid to the
general partners increased $36.7 million for the nine months ended September
30, 1999 as compared to the same period in 1998. The distribution for the
nine months ended


                                      13
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP



September 30, 1999 was impacted by increased earnings as well as a change in
the timing of distribution payments. The distribution for the nine months
ended September 30, 1998 was impacted by a rate case refund during the fourth
quarter of 1997. Financing activities for the nine months ended September 30,
1999, included $197.5 million from the issuance of the Senior Notes, net of
associated debt discounts and issuance costs, and $12.9 million from the
termination of the interest rate forward agreements. Advances under the
Pipeline Credit Agreement, which were primarily used to finance a portion of
the capital expenditures for The Chicago Project, were $82.0 million for the
nine months ended September 30, 1999 as compared to advances under the
Pipeline Credit Agreement of $265.0 million for the same period in 1998.
During the nine months ended September 30, 1999, $255.0 million was repaid on
the Pipeline Credit Agreement.

YEAR 2000

     TC PipeLines, LP and the general partner are not materially dependent
upon computer systems to conduct their businesses. Accordingly, management
does not believe that the Year 2000 issue will have a material adverse effect
on the Partnership's business, financial condition or results of operations,
except as to any material adverse effect that may result from any Year 2000
issue affecting Northern Border, as discussed below.

     Similar to most businesses, Northern Border relies heavily on
information systems technology to operate in an efficient and effective
manner. Much of this technology takes the form of computers and associated
hardware for data processing and analysis. In addition, a great deal of
information processing technology is embedded in microelectronic devices.

     The Year 2000 issue results from the use in computer hardware and
software of two digits rather than four digits to define the applicable year.
As a result, computer programs that have date-sensitive software may
recognize a date using "00" as the year 1900 rather than the year 2000.

     If not corrected, many computer applications could fail or create
erroneous results. The effects of the Year 2000 issue are compounded because
of the interdependence of computer and telecommunication systems in the
United States and throughout the world. This interdependence is true for
Northern Border and its respective suppliers and customers.

     Northern Border has developed a plan, which will be modified as events
warrant, to address Year 2000 issues (the Plan). The Plan is designed to
take reasonable steps to prevent mission-critical functions from being
impaired due to the Year 2000 issue. Mission-critical functions are pipeline
operations, operated in a manner that is safe for personnel and the public.
Pipeline operations include the flow of natural gas through the pipeline with
the operation of thirteen natural gas fired compressor stations, two electric
powered compressor stations, measurement stations for receipt and delivery of
gas and the supervisory control and data acquisition (SCADA) computer
system.

     Northern Border is committed to allocating the resources necessary to
implement the Plan. A core team of individuals has been established to
implement and complete the Plan (the Y2K Team). The Plan includes
developing a comprehensive component inventory of computer hardware,
software, embedded chips and third-party interfaces; assessing the risk of
non-compliance of each component; identifying the impact of any component
failure; assessing Year 2000 compliance of each component; identifying and
implementing solutions for non-compliance of components; testing of solutions
implemented; and developing contingency plans for critical components and
systems.

     Northern Border has identified, inventoried, and assessed computer
software, hardware, embedded chips, and third-party interfaces. Where
necessary, remediation, replacement, or adequate workarounds have been
identified and implemented. The workaround Northern Border has employed in
very limited instances is to turn the system clocks back to a past date so
that the systems will not rollover to the year 2000 for several years.


                                      14
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


     System clocks have been turned back at two compressor stations that have
an older control system that has not been upgraded. TC PipeLines, LP believes
that there is limited risk involved with turning the clocks back on these
computer systems because all functions are local to the station and their
operations are not dependent on knowing the exact date. This strategy allows
the computers to continue to function until replaced.

     At this time, it is TC PipeLines, LP's understanding that all of
Northern Border's mission critical systems are Year 2000 ready. As far as
non-mission critical systems, it is TC PipeLines, LP's understanding that the
systems are over 95% Year 2000 ready, based on the work effort involved.
Additional work remaining consists primarily of completing upgrades of
certain off-the-shelf software.

     The Plan recognizes that the computer, telecommunications and other
systems (Outside Systems) of outside entities (Outside Entities) have the
potential for major, mission-critical, adverse effects on the conduct of
Northern Border's business. Northern Border does not have control of these
Outside Systems. However, the Plan includes an ongoing process of identifying
and contacting Outside Entities whose systems have or may have a substantial
effect on Northern Border's ability to continue to conduct the
mission-critical aspects of their businesses without disruption from Year
2000 issues. The Plan requires Northern Border to attempt to inventory and
assess the extent to which these Outside Systems may not be Year 2000
compatible. The Y2K Team will reasonably attempt to coordinate with these
Outside Entities in an ongoing effort to obtain assurances these Outside
Systems will be Year 2000 compatible well before January 1, 2000.

     A listing of critical Outside Entities has been developed which includes
shippers, electrical suppliers, and interconnecting pipelines. The Y2K Team
has contacted these entities to determine their Year 2000 readiness and the
extent to which joint testing or mutual contingency planning is required. All
critical Outside Entities contacted have Year 2000 readiness programs
underway and they expect to be Year 2000 ready before the end of the year.
The assessment of the Year 2000 readiness of critical Outside Entities is an
important factor in the internal contingency planning process.

     The processes of inventorying, assessing, analyzing, remediating through
replacement or adequate workarounds, testing, and developing contingency
plans for mission-critical functions in anticipation of the year 2000 are
necessarily iterative processes. That is, the steps are repeated as the Y2K
Team learns more about the Year 2000 issue and its effects on internal
systems and on Outside Systems, and about the effects that embedded chips may
have on Northern Border's systems and Outside Systems. As the steps are
repeated, it is likely that new issues will be identified and addressed. TC
PipeLines knows of no specific systems that are susceptible to issues that
can only be identified after January 1, 2000.

     Northern Border has in place a contingency plan designed to address
specific Year 2000 related issues including loss of all commercial electrical
power, loss of all commercial telecommunications, and unforeseen failures in
critical systems. In the event of loss of commercial power, all critical
facilities have back up power sources including auxiliary generators and
battery back up except for the two electric powered compressor stations.
Northern Border has its own internal, private communications systems for
voice, data, and SCADA traffic. All of the communications sites have back-up
power sources. Northern Border also has a redundant back-up site for critical
operation and systems functions. All compressor facilities will be manned at
year-end so that unforeseen issues can be dealt with immediately.

     Northern Border has not incurred material historical costs associated
with the Year 2000 issues. Further, TC PipeLines, LP believes that Northern
Border's future costs of implementing the Plan will not be material. Although
TC PipeLines, LP believes Northern Border's estimates are reasonable, there
can be no assurance, for the reasons stated in the following paragraph, that
the actual costs of implementing the Plan will not differ materially from the
estimated costs or that Northern Border will not be adversely affected by
Year 2000 issues.


                                      15
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONTINUED)

                                TC PIPELINES, LP


     The extent and magnitude of the Year 2000 issue as it may affect
Northern Border, both before and for some period after January 1, 2000, are
difficult to predict or quantify for a number of reasons. Among the most
important is the potential complexity of locating embedded microprocessors
that may be in a great variety of hardware used for process or flow control,
environmental, transportation, access, communications and other systems. TC
PipeLines, LP believes that Northern Border will be able to identify and
remediate mission-critical systems containing embedded microprocessors. Other
important difficulties relate to the lack of control over and difficulty
inventorying, assessing, remediating, verifying and testing Outside Systems;
the complexity of evaluating all software (computer code) internal to
Northern Border that may not be Year 2000 compatible; and the potential
limited availability of certain necessary internal or external resources,
including but not limited to trained hardware and software engineers,
technicians and other personnel to perform adequate remediation, verification
and testing of internal systems or Outside Systems. Year 2000 costs are
difficult to estimate accurately because of unanticipated vendor delays,
technical difficulties, the impact of tests of Outside Systems, and similar
events. There can be no assurance, for example, that all Outside Systems will
be adequately remediated so that they are Year 2000 ready by January 1, 2000,
or by some earlier date, so as not to create a material disruption to
business. If, despite diligent, prudent efforts under the Plan, there are
Year 2000-related failures that create substantial disruptions to Northern
Border's business, the adverse impact could be material. Moreover, the
estimated costs of pursuing the current course of action do not take into
account the costs, if any, that might be incurred as a result of Year
2000-related failures that occur despite implementation of the Plan, as it
may be modified over time.

     In a SEC release regarding Year 2000 disclosures, the SEC stated that
public companies must disclose the most reasonably likely worst case Year
2000 scenario. Analysis of the most reasonably likely worst case Year 2000
scenarios Northern Border may face leads to contemplation of the following
possibilities: widespread failure of electrical, gas, and similar supplies by
utilities serving Northern Border; widespread disruption of the services of
communications common carriers; similar disruption to means and modes of
transportation for Northern Border and its employees, contractors, suppliers,
and customers; significant disruption to Northern Border's ability to gain
access to, and remain working in, office buildings and other facilities; and
the failure of Outside Systems, the effects of which would have a cumulative
material adverse impact on the Northern Border's mission-critical systems.
Among other things, Northern Border could face substantial claims due to
service interruptions, inability to fulfill contractual obligations,
inability to account for certain revenues or obligations or to bill shippers
accurately and on a timely basis, and increased expenses associated with
litigation, stabilization of operations following mission-critical failures,
and the execution of contingency plans. Northern Border could also experience
an inability by shippers to pay, on a timely basis or at all, obligations
owed to Northern Border. Under these circumstances, the adverse effect on
Northern Border, and the diminution of Northern Border's revenues, would be
material, although not quantifiable at this time. Northern Border will
continue to monitor business conditions to assess and quantify material
adverse effects, if any, that result or may result from the Year 2000 issue.

     This discussion under the heading "Year 2000" constitutes a year 2000
readiness disclosure under the Year 2000 Information and Readiness Disclosure
Act. Compliance with the Year 2000 Information and Readiness Disclosure Act
does not limit or otherwise affect any claims or actions under the federal
securities laws.

INFORMATION REGARDING FORWARD LOOKING STATEMENTS

     The statements of this Quarterly Report that are not historical
information are forward looking statements within the meaning of Section 27A
of the Securities Act of 1933 and Section 21E of the Securities Exchange Act
of 1934. Such forward looking statements include the discussions in
"Management's Discussion and Analysis of Financial Condition and Results of
Operations" regarding expected capital contributions to be requested by
Northern Border in the year 2000. Although TC PipeLines believes that its
expectations regarding future events are based on reasonable assumptions
within the bounds of its knowledge of its business, it can give no assurance
that its goals will be achieved or that its expectations regarding future
developments will be realized. Important factors that could cause actual
results to differ materially from those in the forward looking statements
herein include industry results, future demand for natural gas, availability
of supplies of Canadian natural gas, political and regulatory


                                      16
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

                 ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF
            FINANCIAL CONDITION AND RESULTS OF OPERATIONS (CONCLUDED)

                                TC PIPELINES, LP


developments that impact FERC proceedings involving Northern Border, Northern
Border's ability to replace its rate base as it is depreciated and amortized,
competitive developments by Canadian and U.S. natural gas transmission peers,
political and regulatory developments in Canada, conditions of the capital
markets and equity markets, and Northern Border's ability to successfully
implement the Year 2000 plan during the periods covered by the forward looking
statements.


                                      17
<PAGE>

                    PART I. FINANCIAL INFORMATION (CONTINUED)

        ITEM 3. QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

                                TC PIPELINES, LP


     For the three months ended September 30, 1999 and for the period May 28
to September 30, 1999, TC PipeLines, LP has not entered into any forms of
financial instruments that are market risk sensitive, either for trading or
non-trading purposes. Therefore, TC PipeLines, LP is not exposed to any
interest rate risk, market price risk, or foreign exchange risk, except to
the extent that its 30 percent general partner interest in Northern Border
exposes the Partnership to the market risks disclosed below.

     Northern Border's interest rate exposure results from its variable rate
borrowings from commercial banks. To mitigate potential fluctuations in
interest rates, Northern Border attempts to maintain a significant portion of
its debt portfolio in fixed rate debt. Northern Border also uses an interest
rate swap agreement to increase the portion of its fixed rate debt. Since
December 31, 1998, there have not been any material changes to Northern
Border's interest rate exposure.


                                      18
<PAGE>

                           PART II. OTHER INFORMATION

                                TC PIPELINES, LP


ITEM 6.  Exhibits and Reports on Form 8-K

(a)      Exhibits.

         None.

(b)      Reports on Form 8-K

         Current Report dated October 7, 1999, on Form 8-K was filed
         October 8, 1999.

         Current Report dated October 21, 1999, on Form 8-K was filed
         October 21, 1999.


                                      19
<PAGE>

                                   SIGNATURES


     Pursuant to the requirements of the Securities and Exchange Act of 1934,
the Registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.

                                 TC PIPELINES, LP
                                 (a Delaware Limited Partnership)

                                 By: TC PipeLines GP, Inc., its general partner

                                 By:  /s/
                                     ------------------------------------------
Date:  November 10, 1999             Theresa Jang
                                     Controller


                                      20