Acute hypertension was induced in 19 anesthetized cats by the intravenous administration of angiotensin. The caliber of pial arteries was measured by a television image-splitting technique and local cerebral blood flow by the hydrogen clearance technique. As the blood pressure was increased, pail arterioles constricted and cerebral blood flow remained relatively constant, showing that autoregulation of cerebral blood flow was intact. At mean arterial pressures of more than 170 mm Hg arteriolar dilation appeared. In smaller arterioles (initial diameter less than 100 mum) a segmental dilation (the "sausage'string" phenomenon) frequently preceded uniform dilation. This arteriolar dilation was associated with a marked increase in local cerebral blood flow indicating that the upper level of autoregulation had been breached. In no cat was vasospasm or a decrease in blood flow observed during induced hypertension. Hypertension also caused dysfunction of the bloodbrain barrier since, in 17 out of 19 of the cats examined, there was extravasation of protein-bound Evans blue into brain tissue. In only one of the 19 cats subjected to neuropathological analysis was ischemic brain damage identified and this was restricted to minimal ischemic cell change. The results indicate that severe, induced hypertension in cats produces cerebral arteriolar dilation, an increase of cerebral blood flow, and dysfunction of the blood-brain barrier. These observations may be of importance in understanding the pathogenesis of hypertensive encephalopathy.