Tissue water and rCBF from the same area of brain was measured in gerbils with cerebral ischemia. In one experiment we related the severity of ischemia that developed after one hour of carotid occlusion to the amount of edema which formed. In a second experiment brain made ischemic for one hour was reperfused for one hour to assess the effect of reperfusion of ischemic tissue upon edema formation. We identified a critical threshold (10-14 ml X 100g-1 min-1) for the reversibility of the ischemic process, above which edema can resolve upon reperfusion. When postocclusion rCBF was less than 10 ml X 100g-1 min-1, edema was maximal at the end of occlusion and did not resolve with reperfusion. Autoregulation was preserved in ischemic tissue in which the edema process resolved with reperfusion.