Abstract： Objective To investigated the effects of leflunomide (Lef) in combination with cyclosporine (CsA) on IKKcomα/β, NF-kB P65, IkBα, ICAM-1 and NF-kB DNA binding activity in NF-kB signal pathway in mouse to rat cardiac xenografts. Methods NIH mice and Wistar rats served as donors and recipients respectively. Mouse to rat heterotopic heart transp lantation was performed (in the neck). The experiments were divided into 4 groups: CsA was injected imraperitoneally after operation in the CsA group; Lef was given in the Lef group by gastric tube after operation; CsA com-bined with Lef was administered after operation in CsA+Lef group; In the control group, the recipi-ents didn't recei veany medicine treatment. After operation, the survival of the transplanted heart was observed. The histophathologic examination was done in the transplanted heart. The expression of IKKα/β, NF-kB P65, IkBα and ICAM-1 and NF-kB DNA binding activity in cardiac xenograft tissues were determined by immunohistochemistry and Western blot as well as electrophoretic mobility shift assay (EMSA) in each group. Results The survival time of the cardiac xenografts in control group,CsA group, Lef group and CsA+Lef group was (2.17±0.41), (2.50±1.05), (4.17±1.33) and (6.50±2.56) days, respectively, significantly longer in CsA + Lef group than in other three groups(P<0.05), and significantly longer in Lef group than in control group (P<0.05). The expression ofIKKα/β (IOD) in cardiac xenograft tissues in control group, CsA group, Lef group and CsA+Lef group was (137.1±4.0), (149.7±6.5), (111.6±14.6) and (81.4±15. 9), the expression ofNF-kB P65 (IOD) was (261.3±31.6), (263.1±28.8), (173.5±5.9) and (67.8+3.8), theexpression of IkBa (IOD) was (88.6±6.7), (80.4±3.5), (64.8±8.9) and (37.3±4.4), theexpression of ICAMol (IOD) was (155.4±5.3), ( 150.7+5.1), ( 104.7±6.2) and (29.2±2.8),and NF-gB DNA binding activity (IOD) was (234.3±8.7), (227.5±14.8), (110.9±12.5) and(43.6±7.4), respectively. The IODs in CsA + Lef group were markedly lower than in the other threegroups (P<0.05), and those in Lef group was obviously lower than in control group (P<0.05).Conclusion The combined use of Lef and CsA can significantly suppress the expression of IKKα/αβ,NF-kB P65, IkBα, ICAM-1 and NF-kB DNA binding activity, which may obviously prolong thesurvival time of mouse to rat cardiac xenografts.