The effect of middle cerebral arterial occlusion on blood flow over the parietal aspect of the cortex and in the putamen of baboons has been assessed by the technique of hydrogen clearance. Reduction of blood flow was present over the entire lateral aspect of the cerebral hemisphere, maximal in the regions of frontal, parietal and temporal opercula where the blood flow was reduced to some 25% of control levels, and in the deep nuclei where the average flow in three animals was 20%. Reactivity to carbon dioxide was correspondingly reduced and, indeed, paradoxical reactivity or intracerebral steal was found in the most densely ischemic opercular zone. There was no evidence in the present studies of any area with increased tissue blood flow following middle cerebral occlusion.Under control conditions, some inhomogeneity of blood flow was evident in the cortical areas studied, the parietal association areas having a significantly lower blood flow than the more anterior regions of the hemisphere.