Regional cerebral blood flow (CBF) was measured in both middle ectosylvian gyri of ten cats by recording the clearance of molecular hydrogen (H2) with implanted polarized electrodes 125 n in diameter, before and up to seven days after occlusion of the left middle cerebral artery (MCA) with a device implanted in the intact cranium. Five patterns of changes of CBF were recorded with the leftsided electrodes. In eight cats MCA occlusion caused immediate decreases of CBF; in the other two cats CBF values were lowest two days after occlusion, presumably because of ischemic edema. Both persistent severe ischemia and early spontaneous postischemic hyperemia were associated with severe neurological deficits, marked swelling of the left cerebral hemispheres, and large infarcts. Late postischemic hyperemia was associated with less severe deficits, less swelling, and smaller infarcts, but the least severe deficits and smallest infarcts were noted in association with persistent moderate ischemia. No consistent patterns were recorded with the rightsided electrodes in this study. Hyperemia which develops spontaneously or is induced shortly after the onset of cerebral ischemia potentially may be harmful because of secondary increases of cerebral edema.