Objective:To conduct study on the protective effect of Tibetan Rannasangpei Pill administered on a one-time treatment of cerebral ischemia /reperfusion injury and to explore the time-effect relationship.Methods:The focal cerebral ischemia-reperfusion injury model was made by middle cerebral artery occlusion (MCAO) in rats,and the rats cerebral infarction ratio,neural behaviors,mental status,weight,and its biochemical indexes of blood including MDA、 T-SOD、 T NOS、 iNOS、 NO were respectively detected at 2h,6h,12h,24h,48h and 72h after the cerebral ischemia.And the regression analysis was adopted to explore the time-effect relationship.Results:The protective effect of Rannasangpei Pi11(33.5mg/kg/d) is proven on MCAO rats,which can reduce the rate of cerebral infarction and inhibit the weight reducing.The mechanism may be related with the inhibited lipid per-oxidation.There is a clear time-effect relationship in Rannasangpei Pill for the treatment of cerebral ischemia injury,which increases the rate of cerebral infarction at 2h,and significantly decreases the rate of cerebral infarction at 24h.The lowering effect of rats weight has been inhibited for a long time (after ischemia at 2h,6h,24h and 72h).The influence of MDA,T-SOD,TNOS,iNOS and NO in serum mainly occurs during the 2h-48h period.Conclusion:The most obvious time-effect relationship of Rannasangpei Pill for the treatment of cerebral ischemia-reperfusion injury appears at 24h,which is proven to be the best treatment time.This conclusion can be used as a reference for the clinical rational drug use.