In a double-blind study, the effects of a large dose (20 mg per kilogram) and a small dose (2 mg per kilogram) of phenoxybenzamine (PBZ) on cerebral infarction were evaluated in 120 Mongolian gerbils. The left common carotid artery was ligated in 100 animals; a sham operation was done in 20 animals. One hour later, 25 animals were given 2 mg per kilogram of PBZ, 25 animals were given 20 mg per kilogram of phenoxybenzamine, and 50 animals were given 0.5 cc of normal saline, all doses being repeated at 24, 48, and 72 hours. Five sham-operated animals were given 2 mg per kilogram of phenoxybenzamine, five were given 20 mg per kilogram of phenoxybenzamine and ten were given 0.5 cc of normal saline on the same treatment schedule. Morbidity and mortality were recorded for one week and then all surviving animals were killed. All brains were studied for signs of infarction. Of the saline-treated animals, 32% had cerebral infarction and 81% of these died. Of the animals treated with phenoxybenzamine, 36% of those receiving 2 mg per kilogram and 68% (p < 0.05) of those receiving 20 mg per kilogram had cerebral infarction and all of those with infarction died during the observation period. The animals receiving phenoxybenzamine had a larger stroke index than those treated with saline. The authors concluded that phenoxybenzamine is harmful in postischemic treatment of strokes.