EXHIBIT 99.13
                                                                   -------------

                                NOTICE OF DEFAULT


                                December 5, 2000

Greenbriar Corporation
4265 Kellway Circle
Addison, Texas 75244
Attention: Gene S. Bertcher

Gentlemen:

         Reference is made to (i) those certain Certificates of Voting Power,
Designations, Preferences, and Relative, Participating, Optional or Other
Special Rights of Series F Senior Convertible Preferred Stock (the "Series F
Stock") and Series G Senior Non-Voting Convertible Preferred Stock (the "Series
G Stock") of Greenbriar Corporation ("Greenbriar") (collectively, the
"Certificates") and (ii) that certain Stock Purchase Agreement (the "Purchase
Agreement") between Greenbriar and Lone Star Opportunity Fund, L.P. ("LSOF"),
dated as of December 31, 1997. Capitalized terms used herein and not otherwise
defined are used as defined in the Certificates.

         It has come to the attention of the undersigned that Greenbriar has
failed to comply with Section 6.26 of the Purchase Agreement which, among other
things, prohibits Greenbriar from violating (i) the provisions of its articles
of incorporation and (ii) any obligation, provision, condition, covenant or
requirement contained in the Transaction Documents (as defined in the Purchase
Agreement), if that violation or default would result in a material impairment
of the ability of any holder of Preferred Stock (as defined in the Purchase
Agreement) to enforce any of Greenbriar's obligations or any of such holder's
rights in connection with the Preferred Stock. Greenbriar's failure to properly
comply with the Notice of Conversion (the "Notice of Conversion"), dated as of
October 30, 2000 that was delivered by LSOF to Greenbriar constitutes
Greenbriar's failure to comply with Section 6.26 of the Purchase Agreement. As a
result, under the Certificates, an Event of Default has occurred.

         Pursuant to Section 8 of each of the Certificates, upon the occurrence
and during the continuance of an Event of Default, the undersigned is entitled
to: (i) an additional dividend of 12% per annum and (ii) put its Series F Stock
and Series G Stock to Greenbriar for 120% of the Liquidation Value. In addition,
pursuant to Section 8 of the Certificate for the Series F Stock, upon the
occurrence and during the continuance of an Event of Default the holders of the
Series F Stock will be entitled to designate and have appointed a number of
persons to the Board of Directors of Greenbriar that will constitute 70% of the
members thereof.

         The undersigned hereby informs Greenbriar that (i) it expects to be
paid the additional dividends on the Series F Stock and Series G Stock referred
to above from the date that the Event of Default first took place and (ii) that
it intends to designate and have appointed the maximum number of Board members
that it is entitled to under Section 8 of the Certificate for the Series F
Stock. The undersigned will provide Greenbriar with the names of such Board
member designees shortly. The undersigned further informs Greenbriar that it
does not waive any other rights or remedies that it may have arising as a result
of Greenbriar's failure to comply with Section 6.26 of the Purchase Agreement
and as a result of the Event of Default arising therefrom including, without
limitation, the undersigned's right to put the Series F Stock and Series G Stock
to Greenbriar.








                       LSOF POOLED EQUITY, L.P.

                       By:  LSOF GenPar, Inc., Its General Partner


                       By: /s/J.D. Dell
                           ----------------------------------------
                             J.D. Dell
                             Vice President