Objective To investigate the effects of methycobal on the expression of Caspase-3 in brain tissue after cerebral ischemia reperfusion in rats. Methods Rats were randomly divided into sham-operation group, model control group, nimodipine group and low-dose methycobal group, high-dose methycobal group(n=30 in each group).Rats in the sham-operation group and model control group were administered intragastrically with 0.9% sodium chloride solution, rats in the nimodipine group were treated with 1 mg . kg-1 . d-1 of nimodipine, rats in the low- and high-dose of methycobal groups were given 50 and 100 μg.kg-1 .d-1 of methycobal, respectively. The rat model of cerebral ischemia reperfusion was established by middle cerebral artery occlusion with suture method for 3 h.Neurological deficit scores were evaluated 24 h after reperfusion.The apoptosis of perifocal cortex cells was detected by TUNEL method and the expression of Caspase-3 was analyzed by RT-PCR 6, 12 and 24 h after reperfusion. Results Neurological deficit scores in model control group, nimodipine group, low-dose methycobal group and high-dose methycobal group were 2.70±0.52, 1.30±0.51, 2.20±0.75 and 1.30±0.81, respectively.Compared with model control group, neurological deficit scores were significantly different in the nimodipine group, low-dose methycobal group and high-dose methycobal group(P<0.05 or P<0.01).There were no significant differences between the high-dose methycobal group and nimodipine group ( P>0. 05 ) . There was a significant difference between the high-dose methycobal group and low-dose methycobal group( P<0. 05 ) . The results of apoptosis by TUNEL were as follows: compared with model control group, the apoptosis decreased obvsiouly in the nimodipine group, low-dose methycobal group, and high-dose methycobal group at each time point.There was significant difference between the high-dose methycobal group and nimodipine group at the end of the 24th hours (P<0.01).Compared with low-dose methycobal group, there were significant differences in the high-dose methycobal group at the end of 6th, 12th and 24th hours(P<0.01 or P<0.05).The results of RT-PCR were as follows: there was expression of caspase-3 mRNA in the perifocal cortex of all groups, with weak expression in the sham-operation group.Compared with the sham-operation group, the expression of caspase-3 mRNA was increased significantly in the model control group(P<0.01).The expression of caspase-3 mRNA was reduced significantly in the nimodipine group, the low-dose methycobal group and high-dose methycobal group as compared with model control group at each time point( P<0.05 or P<0.01) , but it was not significantly different in the low-dose methycobal group and high-dose methycobal group as compared with that of the nimodipine group(P>0.05).There were significant differences between the high-dose methycobal group and low-dose methycobal group at the end of 24 h(P<0.05). Conclusion Methycobal can protect the brain cells from injury after cerebral ischemia reperfusion by adjusting the expression of Caspase-3m RNA, and the high-dose methycobal is more effective.