Clentiazem reduces infarct size in rabbit middle cerebral artery occlusion. We assessed the value of pretreatment with clentiazem (8-chlorodiltiazem), a diltiazem derivative with cerebroselective properties, on the consequences of surgical occlusion of the middle cerebral artery via a transorbital approach in 38 rabbits. Nineteen rabbits received 1.7 (n = 5), 5 (n = 8), or 15 (n = 6) mg/kg clentiazem orally four times a day for 24 hours before and 48 hours after occlusion. Upon sacrifice, a segment of the right middle cerebral artery distal to the occlusion and a corresponding segment from the nonoccluded left middle cerebral artery were mounted on myographs for in vitro study of their reactivity to histamine, acetylcholine, serotonin, norepinephrine, and electrical stimulation of intramural sympathetic nerves. Morphometric measurements of 2,3,5-triphenyltetrazolium chloride-stained brain slices permitted us to estimate infarct volume. Pretreatment with 1.7, 5, and 15 mg/kg clentiazem significantly reduced infarct volume (p less than 0.05, p less than 0.01, and p less than 0.01, respectively). Mean infarct volume of the 15 mg/kg-treated group was only 4% that of the untreated group. There were no postoperative deaths in any treated group compared with a death rate of 36% in the untreated group. Mean values for vascular smooth muscle contractility to histamine and relaxation to acetylcholine were significantly enhanced in vessels from treated rabbits. These studies indicate that pretreatment with clentiazem offers cerebral protection and significantly reduces infarct volume as well as arterial wall damage beyond the occlusion. 