Cerebral blood flow and metabolism have been studied during hypotension induced with sodium nitroprusside in 20 patients undergoing surgery for cerebral aneurysms. Cerebral blood flow was measured using an injection of xenon-133 into the common carotid artery, after the retrograde cannulation of the superficial temporal artery, and exponential stripping of the decay curve as detected by a scintillation counter placed over the ipsilateral fronto-parietal region. In the group as a whole there was no significant decrease in cerebral blood flow, as determined by measurement of grey matter flow, or change in the cerebral metabolic rate for oxygen when the blood pressure was reduced by 42%. Cerebrovascular resistance decreased by 40%. During hypotension there were significant decreases in arterial oxygen tension, arterial and jugular venous oxygen contents, but insignificant decreases in jugular venous oxygen tension and arterio-jugular venous oxygen content difference. There were no complications that could be attributed to sodium nitroprusside.