Using electron microscopy, we investigated the small arteries and veins in ischemic lesions induced by occlusion of the ostium of the middle cerebral artery in 42 rats. We observed endothelial denudation in the small arteries of rats receiving occlusion for greater than 6 hours. When the occluding cylinder was removed to allow for reperfusion, however, 2 hours of occlusion was sufficient for endothelial denudation to occur. Medial smooth muscle cells seemed to be more vulnerable to ischemia than endothelial cells because ultrastructural changes in the former cells preceded endothelial denudation. Moreover, endothelial denudation definitely exacerbated vascular changes so that medial necrosis appeared to be complete beneath the denuded areas, allowing erythrocytes, platelets, and exogenous tracers to penetrate into the cytoplasm of smooth muscle cells. These arterial lesions seemed to be repaired 10 days after removal of the occluding cylinder following 2 hours of occlusion. On the other hand, small veins in the ischemic lesion did not show endothelial denudation or medial necrosis. Our study suggests that arterial changes in ischemic lesions play a role in exacerbating the brain edema caused by recirculation after ischemia.