The effect of prostaglandin E1 on regional cerebral blood flow (rCBF) was studied with the intra-arterial 133Xe method in ten awake patients under local anesthesia. Measurements were taken from 16 areas of a hemisphere in seven patients, from 35 areas of a hemisphere in two patients and from 256 areas of a hemisphere in one patient. The prostaglandin was dissolved from the crystalline state without the aid of alcohol. It was given intracarotidly as a constant infusion at a rate of 5 ng per kilogram per minute for five minutes before the measurement and continued during the measurement. In every patient a mild increase in blood flow during the prostaglandin infusion was seen. The flow increase took place in all parts of the hemisphere. It averaged 11.2% (p less than 0.01). During the infusion, the skin supplied by the internal carotid artery and the conjunctiva on the infused side became red and sometimes swollen. A slight pressure was noted by most patients, but none had pain. No side effects of the infusion were noted.