Prostaglandin E1(PGE1) was administered for deliberate hypotension during general anaesthesia in 27 patients undergoing cerebral-aneurysm clip ligation, and the effect of the drug on local cerebral blood flow was studied. Local cerebral blood flow measurements were made using a thermal-gradient blood flowmeter. Control measurements were made immediately before administration of the drug. Local cerebral blood flow was measured 10, 30, 60 and 120 min after starting the drug infusion and 10, 30 and 60 min after discontinuation. The mean blood pressure was reduced significantly by the administration of PGE1. Urine output was increased after commencement of the drug infusion. Local cerebral blood flow did not change. These results suggest that PGE1 may be an appropriate hypotensive drug for use during cerebral-aneurysm clip ligation, because the cerebral blood flow remains within the normal range and the urine output is increased.