In order to evaluate extracranial-intracranial arterial anastomosis in acute ischemic stroke, changes in the local cerebral blood flow (lCBF) in the cortical and subcortical areas were investigated by hydrogen clearance during left middle cerebral artery (MCA) occlusion and maxillary artery (MA)-to-MCA anastomosis in 24 mongrel dogs. Following MCA occlusion, the lCBF was maintained at more than 25ml/100g/min in 17 dogs because of effective leptomeningeal collaterals. In seven dogs, cortical ischemia was shown with progressive lCBF reduction to below 25ml/100g/min. Severe persistent ischemia of less than 10ml/100g/min was not observed. Subcortical ischemia with lCBF reductions to below 25ml/100g/min in the gray matter and below 10ml/100g/min in the white matter occurred more frequently than cortical ischemia. Following MA-MCA anastomosis, the cortical lCBF returned to nearly the preocclusion value, and the subcortical lCBF showed a slight or moderate increase. Reactive hyperemia was noted in two dogs (at three sites), but marked hyperemia was not abserved in any of the animals. The increase in the lCBF after the anastomsis showed a linear relationship to the donor arterial supply. Postmortem and histological examinations revealed no cerebral edema or hemorrhagic infarct. Comparative analyses of these results with the concept of "ischemic penumbra" led to the conclusion that cerebral ischemia with the cortical lCBF ranging from 15 to 25ml/100g/min (30 to 50% of the preocclusion value) after MCA occlusion cannot be reversed even by the leptomeningeal collaterals and may produce progessive neurological dysfunction. In such an ischemic condition, rapid restoration of the lCBF by extracranial-intracranial arterial anastomosis might be effective in the prevention of cerebral infarction as well as in the recovery of neuronal function.