The effects of morphine-nitrous oxide anesthesia on cerebral autoregulation were studied in healthy male volunteers. Anesthesia was morphine, 2 mg/kg, and 70 per cent nitrous oxide in oxygen. Ventilation was controlled and carbon dioxide added to keep Paco-2 constant at 40 torr. Cerebral blood flow was measured first at the subject's normal mean arterial blood pressure, than at 60 torr and at 120 torr in a randomly assigned balanced order. Last, in five subjects cerebral blood flow was measured again at normal mean pressure. Blood pressure alteration was accomplished using phenylephrine or trimethaphan. Cerebral blood flow was 38.9 plus or minus 6.4 (SEM) ml/100 g/min at normal mean pressure, 49.5 plus or minus 10.7 ml/100 g/min at 60 torr. These values are not different a P IS LESS THAN 0.05. The data were analyzed for the possible effect of time on cerebral blood flow, and no change could be domonstrated. It is concluded that with Paco-2 constant at 40 torr morphine-nitrous oxide anesthesia does not significantly affect cerebral autoregulation in normal man.