Abstract: Objective To study the effect of heme oxygenase-1(HO-1) on cardiac graft survival in rats and its mechanism. Methods Male LEW (RT1l) and LEW.1W(RT1u) rats were used as donor and recipient of heterotopic heart transplantation,respectively.Recipient rats were treated with CoPPIX (5mg/kg/day，n=6),SnPPIX (5mg/kg/day，n=6)and control(n=6),starting from one day to rejection.Histology,Immunohistochemistry staining ,HO activity assay and Western blot of HO-1 were performed using cardiac grafts.Mixed lymphocyte cultures were completed using irradiated LEW.1W splenocytes with splenocytes derived from cardiac recipients. Grafts were excised from recipient at 5 days after transplantation. Results Induction of HO-1 with CoPPIX resulted in a significant prolongation of graft survival,the median survival time of the grafts was 13.5 days(p<0.05).The median survival time of SnPPIX treatment and control group were 6.5 and 7.0 respectively.In the grafts of control and SnPPIX treatment group,there is significant cellular infiltration.Treatment of CoPPIX induced the cellular infiltrate of CD4+,CD25+,Ki67+ cells. CoPPIX treatment significantly suppressed donor reactive spenocyte proliferation when compared to control and SnPPIX treatment(p<0.05). Conclusions Induction of HO-1 inhibits alloreactive lymphocyte activity and prolongs cardiac graft survival in rats.