The object of this investigation was to study the effects of mannitol and high dose of methylprednisolone(MP)upon evolution of cerebral infarction in cats after acute left middle cerebral artery(MCA) occlusion and following reperfusion. The acute occlusion of left proximal MCA of thirty cats for 2, 4 and 6 hours respectively were accomplished by applying the Heifetz clip through the retroorbital extradural approach and followed by 2 hours of recirculation. Fifteen cats were untreated as a control group and the fifteen cats were given a combination of mannitol(2g/kg) and MP (15mg/kg) at 30 minutes after occlusion initially, and then every one and a half hour. Results of morphologic examination of the brain demonstrated that untreated cats undergoing 2-hour or 4-hour occlusion mannitol and MP improved the ischemic edema and infarction, but in treated cats undergoing 6-hour occlusion they had little protective effect in ischemic brain injury(swelling, neuronal damage and hemorrhagic infarction). Electronecephalography(EEG) in cats undergoing 2-or 4-hour occlusion showed more increasing activities and voltage than in untreated groups, but in cats undergoing 6-hour occlusion low. voltage and slow waves with poor activity, i.e, generally suppressed pattern, were observed and were not significantly different between treated and untreated groups. Our experimental studies of the therapeutic beneficial effects of mannitol and MP were observed in cats of 2-or 4-hour occlusion of MCA with 2 hour recirculation. Therefore, it was suggested that mannitol and MP will prolong the period of potential reversibility of cerebral ischemia following reperfusion within 4 hours of ischemia.