Abstract: Objective To investigate the anticancer activity and apoptosis-regulated mechanisms of bufalin in the orthotopic transplantation model of human colorectal cancer in nude mice. Methods HCT-116 cells of human colorectal cancer were implanted into the colon to establish orthotopic transplantation tumor models of human colorectal cancer in nude mice.Sixty mice were randomly divided into five groups: NS group,5-Fu group,Low dose bufalin group(BL), Medium dose bufalin group(BM) and High dose bufalin group(BH),with 12 mice in each group.Each group was injected intraperitoneally for 7days,respectively.Six mice in each group were killed at d24 and survival time in each group was calculated.The volume of the tumor were measured and the tumor inhibiting rate were calculated.The apoptotic rate measured by TUNEL staining method, and the expression of apoptosis regulated genes Bcl-xL and Bax were detected by real-time fluorogenic quantitative polymerase chain reaction(RFQ-PCR) and Western blot in tumor tissues.ResultThe tumor volume of 5-Fu group and each Bufalin group were reduced significantly compared with NS group (P＜0.01),and the tumor inhibiting rate were 69.6%,45.6%,56.2% and 58.5%for group of 5-Fu,BL,BM and BH respectively.The survival time were prolonged in group BL and BM (P＜0.05).The apoptotic rate in each group of bufalin were increased significantly compared with NS group(P＜0.05).The result of RFQ-PCR and Western blot staining showed that the expression of Bcl-x_L was decreased both in mRNA and protein level, while the expression of Bax was increased. Conclusion Bufalin has significant anticancer activity in the orthotopic transnplantation model of human colorectal cancer in nude mice and it can induce apoptosis of transplanted tumor cells.The mechanism may be associated with the down-regulation of Bcl-x_L and up-regulation of Bax in tumor cells.