Local cerebral blood flow was measured with the hydrogen clearance technique during intravenous infusion of nitroprusside in monkeys. Concentrations of the drug required to reduce the mean arterial blood pressure by less than 40% resulted in no significant change or only a slight increase in regional cerebral blood flow. Larger doses, however, produced a loss of cerebral autoregulation, thereby inducing a drop in cerebral blood flow. Intracranial pressure rose proportionately to the nitroprusside dose during the infusion.