Objective The aim the present study is to examine the expression change oi AQ, P4 on astrocytes and the effect of mild hypothermia on its expression following brain hypoxia/regoxygenation, and to investigate the relationship between cerebral ischemia--reperfusion edema and AQP4 expression, and the mechanism of protection of mild hypothermia on cerebral ischemiareperfusion injury. Methods The primary and subculture cells from the cortex of SD（less than 24 hour old）fetal rats were divided into three groups：control,normal temperature, and mild hypotherma groups. Trypan blue stai- ning was used to detect the astocytes survival rate and changes at different time points after hypoxia/ reoxygenation in 37＂C and 32~C. Cellular damage index was calculated from morphology examination of the cells under inverted phase contrast microscope. [mmunocytochemisca[ assay was carried out to meas- ure the expression change of AQP4 in astrocytes at the indicated time points after hypoxia/reoxygenation and the intervention of mind hypothermia. Results （1） The cell morphology and survival rate were not remarkable at 4 and 8 hours after hypoxia, but reoxygenation resulted in a significant damage. The lon- ger latency,the greater damage was observed in regard to the morphology and the survival rate of astro- eytes. There was lesser damage in mild hypotherma group at corresponding time points. （2）The expres- sion of AQP4 was showed a tendency to decrease at early phase but rising after reoxygenation for 6 h. As time extended it showed a tendency to increase. The expression of AQP4 in normal temperature and mild hypotherma group was significantly lower than in the control group at time points less than or equal to 8 h （P 〈 0.05 or 0.01） ,higher than in the control group atl0 and 12 hrs after reoxygenation （P 〈 0.05 or 0.01）. （3）The expression of AQP4 in mild hypothermia group were significantly lower than in common temperature group at all time points after reoxygenation （P 〈 0. 05 or 0. 01）. Conclusions Astrocytes have stronger tolerance to hypoxia. Mild hypothermia can relieve the deterioration of astrocytes after hypoxiaand reoxygenation and can restrain the expression of AQP4. Restraining AQP4 expressions might be one of the mechanisms underlying the protective effect of mild hypothermia on brain edema.