The object of this investigation was to study the effects of methylprednisolone (M.P.) on the pathological change in the acute focal ischemic cerebral edema in cats. Forty cats were divided 4 groups : The first group was sham control group (4 cats). The second group was induced acute experimental ischemia for 1 hour by occlusion of left middle cerebral artery (M.C.A) applying the Heifetz clip through the transorbital approach under the operating microscope (12 cats). The third and the fourth groups were induced acute experimental ischemia fro 3 hours and 5 hours respectively by the same method (respectively each 12 cats). Each group except sham control group was also divided 3 subgroups : The first, untreated group (4 cats) was recirculated for 2 hours after the acute ischemia. The third, treatment group (4 cats) was recirculated for 2 hours and injected M.P (15mg/kg) at 30 minutes after occlusion initially and then injected at 90 minute interval, respectively. The experimental results are as follows : In cats undergoing 1 or 3 hours of occlusion, treatment with methylprednisolone and recirculation show suppressive effects for subsequent edema and ischemia, but after 5 hours of occlusion, those have little positive or no effect. The effect of treatment with methylprednisolone and recirculation is dependent upon the degree of the ischemic insult and duration of ischemia. The present experimental results suggest that early medical treatment and recirculation within at least 4 hours protect the cerebral tissue from the injurious effect of ischemia. It would be considered that M.P. is used practically in the treatment of acute thromboembolism prevention the irreversible ischemic cerebral edema.