Abstract Objective:To induce cardiac transplantation tolerance by parabiosis in allogeneic rats and study the mechanism of the allograft tolerance.Methods: Spleen cells 2 x 10~8 of DA and LEW rats were injected from ones to the others via the glossal vein.2 day later cyclophos-phamide(CP) 80 mg/kg was injected intraperioneally. On the third day the operation of parabiosis was carried out and then separated after 1 week.The LEW rats received DA cardiac heterotopic allografts.The survival was observed and monitored.The tolerant mechanisms were investi-gated by detection of chimerism,MLR cell-transferred tests in vivo and IL-2 reverse.Results:The cardiac allograft survival was significantly pro-longed; The detection of chimerism showed that the formation of tolerance is correlative to chimerism. MLR and cell-transferred tests in vivo demonstrated that the inhibited immune response of recipient rats is specific for donors, and suppressie cell exists in the recipient rats. The exog-enous IL-2 reverse showed that the tolerance mechanisms are relative to energy. Conclusion: The parabiosis can induce tolerance in rats and prolong rats cardiac allograft survival.The transplantation tolerance include several mechanisms.