The distribution of regional cerebral blood flow (rCBF) within the cerebral hemispheres of rats was investigated following an hypoxic-ischemic insult consisting of a 30-minute exposure to 6.5% to 7% inspired oxygen coupled with unilateral ligation of the common carotid artery and maintenance of normal blood pressure (modified Levine preparation). rCBF was estimated by means of an autoradiographic method employing 14C-antipyrine. Mean arterial PO2 values of 26.8 to 27.5 mm Hg were attained during the insult period. rCBF rose above control values in all structures of the hemisphere contralateral to carotid artery ligation. Structures of the hemisphere ipsilateral to arterial ligation exhibited rCBF values less than those of the opposite side. However, the degree to which carotid artery occlusion reduced the rCBF of a structure relative to that of the opposite hemisphere varied greatly from region to region, with the greatest decrements occurring in the lateral cerebral cortex and the caudoputamen. Previous studies in this model have shown that these regions of the hemisphere ipsilateral to carotid artery occlusion are zones of frequent histological injury. These data suggest that inhomogeneities of rCBF may in part determine the phenomenon of "selective vulnerability" of the nervous system to hypoxia-ischemia.