A possible correlation between changes in local cerebral blood flow (lCBF) and histological changes was examined using the middle cerebral artery (MCA) occlusion model in 19 adult dogs. Local CBF was measured by the hydrogen clearance method. The most proximal portion of the left MCA was clipped through a transorbital approach. The animals were divided into two groups according to the duration of clipping. Group I (subacute group) consisted of 8 dogs in which an anastomosis of a maxillary artery to a middle cerebral artery branch was made seven days after the occlusion of the MCA. Group Ⅱ (acute group) consisted of 11 dogs in which circulation to the brain was restored by releasing the clip which was applied on the MCA for 4 hours. In group I, the average lCBF value in the cortex.rose from 34.2±9.2ml/100g/min before anastomosis to 64.7±24.1ml/100g/min following anastomosis. The average lCBF value in the subcortical regions before anastomosis was 37.4±12.6ml/100g/min, and the value increased to 53.8±11.5ml/100g/min following anastomosis. Hemorrhagic infarction was found by histological examination in 6 out of the 8 dogs. The hemorrhage was superimposed in the infarcted area only in the 6 dogs whose lCBF value was raised above 55ml/100g/min by the anastomosis. The cause of such hemorrhagic infarction may be related to a raised perfusion pressure of the infarcted region with increased vascular vulnerability. In group Ⅱ, a filling defect in the cortex adjacent to the sylvian fissure was observed in fluorescein angiograms (FAG) after MCA occlusion. Recirculation was obtained following release of the clip. The average lCBF value in the cortex with a filling defect after MCA occlusion was 42.3±5.9ml/100g/min, 24.1±11.6ml/100g/min and 42.7±18.8ml/100g/min before, during, and after the 4-hour occlusion of the MCA, respectively. Values of lCBF below 12ml/100g/min were rare during occlusion, and continued up to an hour at most. The usual values of lCBF were above 15ml/100g/min. The histological examination showed neither edema nor hemorrhage. The critical value of lCBF indicating safe revascularization after a 4-hour MCA occlusion is likely to be about 15ml/100g/min.