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Changes in regional cerebral blood flow were correlated with the distribution of histopathologic signs of brain injury in 35 rats after middle cerebral artery occlusion. Rats were allowed to survive for periods of up to 4 weeks after the operation, and we focused particular interest on the time course of blood flow changes from the initial ischemic events to the late stage of infarction. Regional blood flow was measured using [14C]iodoantipyrine and a quantitative autoradiographic technique. Blood flow in regions with histologic signs of infarction (i.e., the lateral caudoputamen and adjacent neocortex) was below 0.238 ml/g/min, corresponding to 15% of normal values for those regions. In perifocal regions without infarction such as the medial caudoputamen and globus pallidus, cerebral blood flow was also reduced, but it never declined below 20% of its normal value. The decrease in cerebral blood flow was most marked during the first hours after occlusion. Thereafter, cerebral blood flow values gradually normalized, and at 4 weeks there were no significant differences compared with the contralateral side. The border between cortical regions with hypoperfusion and normal cerebral blood flow was rather sharp in the coronal plane, but in the sagittal plane there was a more gradual transitional region. The region with hypoperfusion, observed in the sagittal plane, was most widespread in the acute stage, and normalization of flow occurred particularly from anterior and posterior cortical regions toward the ischemic focus. The possibility for penumbral conditions in the cortex thus exists, particularly in the anterior and posterior borders of the infarction, and remains for several hours after the initial insult. Signs of hyperperfusion were present as an early phenomenon in the globus pallidus and caudoputamen, probably because of altered neuronal activity in line with previous studies or caused by hemodynamic changes in the territory of the occluded middle cerebral artery. Late hyperperfusion was seen in the same regions and also in the neocortex from 18 hours to 7 days after occlusion of the middle cerebral artery. Some of these areas with late hyperperfusion showed histologic signs of definite ischemic changes. The changes in cerebral blood flow occurring in and around a focus of cerebral ischemia have thus been determined in rats with occlusion of the middle cerebral artery. By observing the alterations over a long period of time, we have been able to identify the possibility of penumbral conditions in the early phase and the occurrence of reactive hyperemia in the late stage.