Regional cerebral blood flow (rCBF) was studied using the radiolabeled microsphere technique in a canine model of hemispheric ischemia that others have previously examined morphologically with carbon perfusion. It was our goal to ascertain whether this ischemic model, which involves easily accessible occlusions of the ipsilateral circle of Willis, could produce reproducible and significant reduction of hemispheric cerebral blood flow. Seven animals underwent the surgical procedure with measurements of rCBF at base line, after arterial microdissection and brain retraction only, and finally after creation of the lesion. Simultaneous somatosensory evoked potential recordings were also obtained. Bilateral symmetrical flow decreases were referable to anesthetic normalization and maintenance alone before creation of the lesion. The arterial occlusions, however, produced further significant decreases in flow on the ipsilateral side only, sparing all contralateral structures and sparing the brain stem and cerebellum bilaterally. Evoked responses, which had shown no changes in latency or amplitude after dissection and retraction procedures, were completely abolished 15 minutes after the arterial occlusion. Control animals that underwent surgical positioning and anesthesia alone did not show concomitant decreases is rCBF. This model of open craniotomy and direct vascular occlusion is suitable for studies of cerebral ischemia where the therapeutic intervention proposed (such as cerebral revascularization) involves similar surgical manipulations. By this method, a standard and reproducible ischemic lesion is achieved through the surgical field without the need for exposure of the basilar artery.