Background Although some studies have reported that aquaporin-4 （AQP4） plays a role in the post-ischemic edema formation and diffusion-weighted imaging （DWI）, little is known about the AQP4 expression in stage of the reperfusion following acute cerebral ischemia, as well as the correlation between histopathology and DWl. The aim of the study was to investigate the correlation among DWl, histopathology and the AQP4 expression in the reperfused rat brain tissues following acute ischemia.Methods Seventy Wistar rats were randomly divided into a control group （group A）, and several occluded and reperfusion groups. They had their middle cerebral artery unilaterally occluded （MCAO） for 30 minutes （group B） followed by 30 minutes （group D） or 60 minutes （group E） of reperfusion, or 60 minutes of MCAO （group C） followed by 30 minutes （group F）, or 60 minutes （group G） of reperfusion （n=10 for each group）. All rats underwent DWl scanning.The relative apparent diffusion coefficient （rADC） value of each rat was calculated. All the rats were sacrificed and the cerebral ischemic tissues were examined for histopathology. Real-time fluro-quantitative polymerase chain reaction （RT-PCR） and Western-blotting were performed. The amount of AQP4 mRNA （Ex △△Ct） and AQP4 protein （Q） was statistically analyzed. The correlation between rADC values and AQP4 mRNA expression was analyzed with the Pearson correlation test.Results In all the reperfusion groups, the areas of hyper-intensity signal in DWl were decreased, and the rADC value increased and the AQP4 expression decreased significantly compared with the occluded group （t=26.89, t=18.26, P〈0.01）. There was a negative correlation between AQP4 mRNA expression and rADC values （r=-0.72, P〈0.01）. A mixed edema, composed of cerebral intracelluar edema and vasogenic brain edema, was observed in all the reperfusion groups.It was more prevalent in groups D and F than in the groups E and G. With the reperfusion time post
