Objective To evaluate the effects of mild hypothermia performed at different times on the levels of glutamate, Bcl-2 and Bax during global cerebral ischemia-reperfsusion (I/R) in rats. Methods Twenty-four male SD rats weighing 230-270 g were randomly divided into 4 groups according to the time at which mild hypothermia was performed ( n =6 each): group A, B, C and D. Global cerebral ischemia was produced by occlusion of 4 vessels (cauterization of bilateral vertebral arteries and occlusion of bilateral common carotid arteries) in the 4 groups. In group B, C and D, the nasopharyngeal temperature was reduced to 32.5-33.5 ℃ and maintained for 1 h. Ischemia was performed after termination of cooling in group B. While ischemia was performed, cooling was started in group C. While reperfusion was performed, cooling was started in group D. Rewarming was started after termination of cooling in group B, C and D. Samples of dialysate in hippocampal CA1 area were collected every 10 min during 100 min reperfusion for determination of glutamate concentrations by high-performance capillary electrophoresis with laser-induced fluorescence detection ( HPCE-LIF). The brain tissues were taken at 3 h of reperfusion for determination of the expression of Bax and Bcl-2 in hippocampal CA1 area, and the ratio of Bcl-2/Bax was calculated. Results Compared with group A, the glutamate concentrations were significantly decreased, the Bcl-2 expression was up-regulated, the Bax expression was down-regulated and the ratio of Bcl-2/Bax was increased in the other thee groups ( P ＜ 0.05). Compared with group B, the Bcl-2 expression was significantly down-regulated, the Bax expression was up-regulated and the ratio of Bcl-2/Bax was decreased in group C ( P ＜0.05). The glutamate concentrations were significantly increased, the Bax expression was up-regulated and the ratio of Bcl-2/Bax was decreased in group D compared with group C ( P ＜ 0.05 ). Conclusion The earlier cooling is performed, the better the cerebral protective effect is during global cerebral I/R in rats.