BACKGROUND: Aquaporin-4 (AQP-4) over-expression following cerebral ischemia results in cerebral edema. Picroside Ⅱ has been shown to exhibit a neuroprotective effect on neuronal apoptosis. However, few reports have addressed the neuroprotective mechanisms and therapeutic times following cerebral ischemic reperfusion injury. OBJECTIVE: To explore the neuroprotective effects and ideal treatment window for picroside II treatment of middle cerebral artery occlusion and reperfusion injury in rats. DESIGN, TIME AND SETTING: A randomized, controlled, animal experiment was performed at Institute of Cerebrovascular Diseases, Qingdao University Medical College from September 2008 to May 2009.MATERIALS: Picroside II was purchased from Tianjin Kuiqing Medical Technology, China.METHODS: A total of 165 adult, healthy, male, Wistar rats were randomly assigned to sham-surgery (n = 15), model (n = 75), and treatment groups (n = 75). Rats in the model and treatment groups underwent middle cerebral artery occlusion and reperfusion through the use of an intraluminal monofilament suture on the left external-internal carotid artery. The treatment group was injected with 1.0% picroside II (10 mg/kg) into the tail vein, and the model and sham-surgery groups were injected with 0.1 mol/L phosphate buffered saline (250 μL). MAIN OUTCOME MEASURES: Neurological functional scores were evaluated using the Longa's method; cerebral infarction volume was detected through the use of tetrazolium chloride staining; cellular apoptosis was determined through the use of the in situ end-labeling method; aquaporin-4 expression was measured using fluorescence labeling analysis and reverse transcription polymerase chain reaction technique.RESULTS: At 0.5 hour following cerebral ischemic injury, neurological functional scores were low, and a small infarction focus was detected in the ischemic cortex of the model group. Along with prolonged ischemia and an increased number of apoptosis-positive cells, AQP-4 mRNA and protein expression was increased. At 1-2 hours after ischemia, neurological scores and infarction sizes were significantly increased in the model group. Apoptotic-positive cells were widespread in the ipsilateral cortex and striatum. In addition, AQP-4 mRNA and protein expression levels were increased. Picroside II treatment significantly decreased neurological scores and infarction volume, and reduced AQP-4 mRNA and protein expression levels compared with the model group (P < 0.05 or P < 0.01). At 1 hour after ischemia, the therapeutic effect of picroside II was notable (P < 0.01). CONCLUSION: Picroside II played a protective role in cerebral ischemic reperfusion injury by inhibiting apoptosis and regulating AQP-4 expression. The best therapeutic time window was 1 hour after cerebral ischemic reperfusion.