Document:

<PAGE>   1
                                                                   Exhibit 10-CO

                                 AMENDMENT NO. 1

                                     TO THE

                                U.S. $450,000,000

                  AMENDED AND RESTATED 364-DAY CREDIT AGREEMENT

                           DATED AS OF MARCH 10, 1999

                  THIS AMENDMENT NO. 1, dated as of February 15, 2000 (this
"Amendment No. 1") to the Amended and Restated 364-Day Credit Agreement, dated
as of March 10, 1999 (the "Agreement") among Columbia Energy Group, a Delaware
corporation (the "Borrower"), the banks, financial institutions and other
institutional lenders parties to the Agreement (collectively, the "Lenders"),
Salomon Smith Barney Inc., as arranger and book manager, and Citibank N.A., as
administrative and syndication agent for the Lenders, evidences the agreement of
the parties as follows:

                  WHEREAS, the Borrower and the Required Lenders desire to amend
the Agreement in accordance with Section 8.01 thereof.

                  NOW, THEREFORE, in consideration of the foregoing and the
respective covenants, agreements and conditions hereinafter set forth, and
intending to be legally bound hereby, the parties hereto agree as follows:

                  SECTION 1. Amendment to Agreement.

                  Section 6.01(h) of the Agreement is amended to delete the
following text therefrom:

                  "(iii) any Person or two or more Persons acting in concert
                  shall have acquired by contract or otherwise (excluding
                  employment contracts with officers of the Borrower) or shall
                  have entered into a contract or arrangement (excluding
                  employment contracts with officers of the Borrower), that,
                  upon consummation, will result in its or their acquisition of
                  the power to exercise, directly or indirectly, a controlling
                  influence over the management or policies of the Borrower; or"

                  SECTION 2. Capitalized Terms. Capitalized terms used but not
otherwise defined in this Amendment No. 1 shall have the respective meanings
ascribed to such terms in the Agreement.
<PAGE>   2
                  SECTION 3. Execution in Counterparts. This Amendment No. 1 may
be executed in any number of counterparts and by different parties hereto on
separate counterparts, each of which when so executed shall be deemed to be an
original and all of which taken together shall constitute but one and the same
agreement. Delivery of an executed counterpart of a signature page to this
Amendment No. 1 by telecopier shall be effective as delivery of a manually
executed counterpart of this Amendment No. 1.

                  SECTION 4. Governing Law. This Amendment No. 1 shall be
governed by, and construed in accordance with the laws of the State of New York.

                  SECTION 5. Jurisdiction, Etc. (a) Each of the parties hereto
hereby irrevocably and unconditionally submits, for itself and its property, to
the nonexclusive jurisdiction of any New York State court or federal court of
the United States of America sitting in New York City, and any appellate court
for any thereof, in any action or proceeding arising out of or relating to this
Agreement or the Notes, or for recognition or enforcement of any judgment, and
each of the parties hereto hereby irrevocably and unconditionally agrees that
all claims in respect of any such action or proceeding may be heard and
determined in any such New York State court or, to the extent permitted by law,
in such federal court. Each of the parties hereto agrees that a final judgment
in any such action or proceeding shall be conclusive and may be enforced in
other jurisdictions by suit on the judgment or in any other manner provided by
law. Nothing in this Agreement shall affect any right that any party may
otherwise have to bring any action or proceeding relating to this Agreement or
the Notes in the courts of any jurisdiction.

                  (b) Each of the parties hereto irrevocably and unconditionally
waives, to the fullest extent it may legally and effectively do so, any
objection that it may now or hereafter have to the laying of venue of any suit,
action or proceeding arising out of or relating to this Agreement or the Notes
in any New York State or federal court. Each of the parties hereto hereby
irrevocably waives, to the fullest extent permitted by law, the defense of an
inconvenient forum to the maintenance of such action or proceeding in any such
court.

                  SECTION 6. Severability of Provisions. Any provision of this
Agreement which is prohibited or unenforceable in any jurisdiction shall, as to
such jurisdiction, be ineffective to the extent of such prohibition or
unenforceability without invalidating the remaining provisions hereof or
affecting the validity or enforceability of such provision in any other
jurisdiction.
<PAGE>   3
                  IN WITNESS WHEREOF, the parties hereto have caused this
Amendment No. 1 to be executed by their respective officers thereunto duly
authorized, as of the date first above written.

                                   COLUMBIA ENERGY GROUP

                                   By:      /s/ Michael W. O'Donnell
                                            ______________________________
                                            Name:  Michael W. O'Donnell
                                            Title: Senior Vice President and
                                                   Chief Financial Officer

                                   SALOMON SMITH BARNEY INC.,
                                         as Arranger and Book Manager

                                   By:      /s/ J. Christopher Lyons
                                            ______________________________
                                            Name:  J. Christopher Lyons
                                            Title: Attorney-in-Fact

                                   CITIBANK, N.A.,
                                         as Administrative and Syndication Agent

                                   By:      /s/ J. Christopher Lyons
                                            ______________________________
                                            Name:  J. Christopher Lyons
                                            Title: Attorney-in-Fact

                           Lenders

                                   CITIBANK, N.A.

                                   By:      /s/ J. Christopher Lyons
                                            ______________________________
                                            Name:  J. Christopher Lyons
                                            Title: Attorney-in-Fact

                                   THE CHASE MANHATTAN BANK

                                   By:      /s/ Steven Wood
                                            ______________________________
                                            Name:  Steven Wood
                                            Title: Vice President
<PAGE>   4
                                   PNC BANK, NATIONAL ASSOCIATION

                                   By:      /s/ Thomas A. Majeski
                                            ______________________________
                                            Name:  Thomas A. Majeski
                                            Title: Vice President

                                   BANK OF MONTREAL

                                   By:      /s/ Ian M. Plester
                                            ______________________________
                                            Name:  Ian M. Plester
                                            Title: Director

                                   CANADIAN IMPERIAL BANK OF COMMERCE

                                   By:      /s/ M. Beth Miller
                                            ______________________________
                                            Name:  M. Beth Miller
                                            Title: Authorized Signatory

                                   FIRST UNION NATIONAL BANK

                                   By:      /s/ Michael J. Kolosowski
                                            ______________________________
                                            Name:  Michael J. Kolosowski
                                            Title: Vice President

                                   BANKERS TRUST COMPANY

                                   By:
                                            ______________________________
                                            Name:
                                            Title:

                                   BANK ONE

                                   By:      /s/ George R. Schanz
                                            ______________________________
                                            Name:  George R. Schanz
                                            Title: First Vice President

<PAGE>   5
                                   ALLFIRST BANK

                                   By:      /s/ Shaun E. Murphy
                                            ______________________________
                                            Name:  Shaun E. Murphy
                                            Title: Senior Vice President

                                   NATIONAL CITY BANK

                                   By:      /s/ Gregory D. Miller
                                            ______________________________
                                            Name:  Gregory D. Miller
                                            Title: Vice President

                                   COMMERZBANK AG,
                                            NEW YORK BRANCH

                                   By:      /s/ Andrew Kjoller
                                            ______________________________

                                             /s/ Timothy Chin
                                            ______________________________
                                            Name:  Andrew Kjoller
                                                   Timothy Chin
                                            Title: Assistant Vice President
                                                   Assistant Vice President

                                   NATIONSBANK, N.A.

                                   By:      /s/ Paula Z. Kramp
                                            ______________________________
                                            Name:  Paula Z. Kramp
                                            Title: Principal

                                   ARAB BANK, PLC

                                   By:      ______________________________
                                            Name:
                                            Title:

                                   THE BANK OF NOVA SCOTIA

                                   By:      /s/ F.C.H. Ashby
                                            ______________________________
                                            Name:  F.C.H. Ashby
                                            Title: Senior Manager
                                                   Loan Operations
<PAGE>   6
                                   BANK OF TOKYO-MITSUBISHI
                                            TRUST COMPANY

                                   By:      /s/ Nicholas R. Battista
                                            ______________________________
                                            Name:  Nicholas R. Battista
                                            Title: Vice President

                                   CRESTAR BANK

                                   By:      /s/ Nancy R. Petrash
                                            ______________________________
                                            Name:  Nancy R. Petrash
                                            Title: Senior Vice President

                                   BANCA MONTE DEI PASCHI DI SIENA, S.p.A.

                                   By:      ______________________________
                                            Name:
                                            Title:

                                   SOCIETE GENERALE

                                   By:      /s/ Gordon R. Eadon
                                            ______________________________
                                            Name:  Gordon R. Eadon
                                            Title: Director

                                   UNION BANK OF CALIFORNIA

                                   By:      /s/ David Musicant
                                            ______________________________
                                            Name:  David Musicant
                                            Title: Vice President<PAGE>   1
                                                                   Exhibit 10-CP

                                 AMENDMENT NO. 1

                                     TO THE

                                U.S. $900,000,000

                                CREDIT AGREEMENT

                           DATED AS OF MARCH 11, 1998

                  THIS AMENDMENT NO. 1, dated as of February 15, 2000 (this
"Amendment No. 1") to the Credit Agreement, dated as of March 11, 1998 (the
"Agreement") among Columbia Energy Group, a Delaware corporation (the
"Borrower"), the banks, financial institutions and other institutional lenders
listed on the signature page of the Agreement (collectively, the "Lenders") and
Citibank N.A., as administrative and syndication agent for the Lenders,
evidences the agreement of the parties as follows:

                  WHEREAS, the Borrower and the Required Lenders desire to amend
the Agreement in accordance with Section 8.01 thereof.

                  NOW, THEREFORE, in consideration of the foregoing and the
respective covenants, agreements and conditions hereinafter set forth, and
intending to be legally bound hereby, the parties hereto agree as follows:

                  SECTION 1. Amendment to Agreement.

                  Section 6.01(h) of the Agreement is amended to delete the
following text therefrom:

                  "(iii) any Person or two or more Persons acting in concert
                  shall have acquired by contract or otherwise (excluding
                  employment contracts with officers of the Borrower) or shall
                  have entered into a contract or arrangement (excluding
                  employment contracts with officers of the Borrower), that,
                  upon consummation, will result in its or their acquisition of
                  the power to exercise, directly or indirectly, a controlling
                  influence over the management or policies of the Borrower; or"

                  SECTION 2. Capitalized Terms. Capitalized terms used but not
otherwise defined in this Amendment No. 1 shall have the respective meanings
ascribed to such terms in the Agreement.
<PAGE>   2
                  SECTION 3. Execution in Counterparts. This Amendment No. 1 may
be executed in any number of counterparts and by different parties hereto on
separate counterparts, each of which when so executed shall be deemed to be an
original and all of which taken together shall constitute but one and the same
agreement. Delivery of an executed counterpart of a signature page to this
Amendment No. 1 by telecopier shall be effective as delivery of a manually
executed counterpart of this Amendment No. 1.

                  SECTION 4. Governing Law. This Amendment No. 1 shall be
governed by, and construed in accordance with the laws of the State of New York.

                  SECTION 5. Jurisdiction, Etc. (a) Each of the parties hereto
hereby irrevocably and unconditionally submits, for itself and its property, to
the nonexclusive jurisdiction of any New York State court or federal court of
the United States of America sitting in New York City, and any appellate court
for any thereof, in any action or proceeding arising out of or relating to this
Agreement or the Notes, or for recognition or enforcement of any judgment, and
each of the parties hereto hereby irrevocably and unconditionally agrees that
all claims in respect of any such action or proceeding may be heard and
determined in any such New York State court or, to the extent permitted by law,
in such federal court. Each of the parties hereto agrees that a final judgment
in any such action or proceeding shall be conclusive and may be enforced in
other jurisdictions by suit on the judgment or in any other manner provided by
law. Nothing in this Agreement shall affect any right that any party may
otherwise have to bring any action or proceeding relating to this Agreement or
the Notes in the courts of any jurisdiction.

                  (b) Each of the parties hereto irrevocably and unconditionally
waives, to the fullest extent it may legally and effectively do so, any
objection that it may now or hereafter have to the laying of venue of any suit,
action or proceeding arising out of or relating to this Agreement or the Notes
in any New York State or federal court. Each of the parties hereto hereby
irrevocably waives, to the fullest extent permitted by law, the defense of an
inconvenient forum to the maintenance of such action or proceeding in any such
court.

                  SECTION 6. Severability of Provisions. Any provision of this
Agreement which is prohibited or unenforceable in any jurisdiction shall, as to
such jurisdiction, be ineffective to the extent of such prohibition or
unenforceability without invalidating the remaining provisions hereof or
affecting the validity or enforceability of such provision in any other
jurisdiction.
<PAGE>   3
                  IN WITNESS WHEREOF, the parties hereto have caused this
Amendment No. 1 to be executed by their respective officers thereunto duly
authorized, as of the date first above written.

                                     COLUMBIA ENERGY GROUP

                                     By:      /s/ Michael W. O'Donnell
                                              ______________________________
                                              Name:  Michael W. O'Donnell
                                              Title: Senior Vice President
                                                     Chief Financial Officer

                                     SALOMON SMITH BARNEY INC.
                                              as Arranger and Book Manager

                                     By:      /s/ J. Christopher Lyons
                                              ______________________________
                                              Name:  J. Christopher Lyons
                                              Title: Attorney-in-Fact

                                     CITIBANK, N.A.
                                        as Administrative and Syndication Agent

                                     By:      /s/ J. Christopher Lyons
                                              ______________________________
                                              Name:  J. Christopher Lyons
                                              Title: Attorney-in-Fact

                             Lenders

                                     CITIBANK, N.A.

                                     By:      /s/ J. Christopher Lyons
                                              ______________________________
                                              Name:  J. Christopher Lyons
                                              Title: Attorney-in-Fact

                                     THE CHASE MANHATTAN BANK

                                     By:      /s/ Steven Wood
                                              ______________________________
                                              Name:  Steven Wood
                                              Title: Vice President
<PAGE>   4
                                     PNC BANK, NATIONAL ASSOCIATION

                                              By:  /s/ Thomas A. Majeski
                                              ______________________________
                                              Name:  Thomas A. Majeski
                                              Title: Vice President

                                     BANK OF MONTREAL

                                              By:  /s/ Ian M. Plester
                                              ______________________________
                                              Name:  Ian M. Plester
                                              Title: Director

                                     CANADIAN IMPERIAL BANK OF COMMERCE

                                              By:  /s/ M. Beth Miller
                                              ______________________________
                                              Name:  M. Beth Miller
                                              Title: Authorized Signatory

                                     FIRST UNION NATIONAL BANK

                                              By:  /s/ Michael J. Kolosowsky
                                              ______________________________
                                              Name:  Michael J. Kolosowsky
                                              Title: Vice President

                                     BANKERS TRUST COMPANY

                                              By:
                                              ______________________________
                                              Name:
                                              Title:

                                     BANK ONE

                                              By:    /s/ George R. Schanz
                                              ______________________________
                                              Name:  George R. Schanz
                                              Title: First Vice President
<PAGE>   5
                                     MORGAN GUARANTY
                                              TRUST COMPANY OF NEW YORK

                                              By:    /s/ Robert Bottamedi
                                              ______________________________
                                              Name:    Robert Bottamedi
                                              Title:   Vice President

                                     ALLFIRST BANK

                                              By:    /s/ Shaun E. Murphy
                                              ______________________________
                                              Name:  Shaun E. Murphy
                                              Title: Senior Vice President

                                     NATIONAL CITY BANK

                                              By:    /s/ Gregory D. Miller
                                              ______________________________
                                              Name:  Gregory D. Miller
                                              Title: Vice President

                                     COMMERZBANK AG,
                                              NEW YORK BRANCH

                                              By:    /s/ Andrew Kjoller
                                                         Timothy Chin
                                              ______________________________
                                              Name:  Andrew Kjoller
                                                     Timothy Chin
                                              Title: Assistant Vice President

                                     NATIONSBANK, N.A.

                                              By:    /s/ Paula Z. Kramp
                                              ______________________________
                                              Name:  Paula Z. Kramp
                                              Title: Principal

                                     ARAB BANK, PLC

                                              By:
                                              ______________________________
                                              Name:
                                              Title:
<PAGE>   6
                               THE BANK OF NOVA SCOTIA

                                        By:    /s/ F.C.H. Ashby
                                        ______________________________
                                        Name:  F.C.H. Ashby
                                        Title: Senior Manager Loan Operations

                                BANK OF TOKYO-MITSUBISHI
                                        TRUST COMPANY

                                        By:    /s/ Nicholas R. Battista
                                        ______________________________
                                        Name:  Nicholas R. Battista
                                        Title: Vice President

                               CREDIT AGRICOLE

                                        By:
                                        ______________________________
                                        Name:
                                        Title:

                                CRESTAR BANK

                                        By:    /s/ Nancy R. Petrash
                                        ______________________________
                                        Name:  Nancy R. Petrash
                                        Title: Senior Vice President

                               BANCA MONTE DEI PASCHI DI SIENA, S.p.A.

                                        By:
                                        ______________________________
                                        Name:
                                        Title:

                               SOCIETE GENERALE

                                        By:    /s/ Gordon R. Eadon
                                        ______________________________
                                        Name:  Gordon R. Eadon
                                        Title: Director
<PAGE>   7
                                     UNION BANK OF CALIFORNIA

                                              By:    /s/ David Musicant
                                              ______________________________
                                              Name:  David Musicant
                                              Title: Vice President

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