In this study, 12 dogs were anesthetized with sodium pentobarbital, and blood flows were determined using the radioactive microsphere technique. Ten dogs were first made acutely hypertensive by the infusion of norepinephrine and demonstrated preserved cerebral autoregulation. The administration of naloxone, 10 mg/kg i.v., in these animals produced a significant increase in cerebral blood flow and a proportional drop in cerebrovascular resistance with no change in the cerebral metabolic rate of oxygen or the electroencephalogram. Two additional spontaneously hypertensive dogs demonstrated a similar response to naloxone. These results suggest that high dose naloxone produces cerebrovasodilation either directly or through the inhibition of cerebral autoregulation.