Cerebral blood flow (CBF) in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of CBF over several hours. Morphine and the opiate antagonist, naloxone, were tested for their effects on the reactive hyperemia that follows a brief anoxic challenge. Morphine (5.0 mg/kg) significantly reduced the peak increase in flow during the hyperemia and, at both of the doses used (1.0 and 5.0 mg/kg), caused a small, nonsignificant increase in the duration of the reactive hyperemia. Naloxone (0.1 and 1.0 mg/kg) enhanced basal CBF rates and significantly prolonged the duration of the reactive hyperemia. These effects of naloxone may account for its beneficial effects in the treatment of cerebral ischemia.