The effect of naloxone, opiate antagonist, was investigated on systemic arterial blood pressure(BP), cerebral blood flow (CBF), and cerebral oxygen consumption in the cat brain following occlusion of the middle cerebral artery(MCA). A total of 21 adult cats was studied under the anesthesia of alpha-chloralose (50 mg/kg). The left MCA was exposed and coagulated using transorbital approach under a operation microscope. Naloxone of 5 mg/kg was administrated intravenously within 30 minutes after MCA occlusion in early therapy group and 2.5 hours after the occlusion in delayed therapy group. Isotonic saline was administrated in the control group. The basal value of CBF in the intact animals was 54.0 +/- 5.9 (+/- SEM) ml/100g/min. Significant increases in mean BP of 13%, CBF of 12%, and CMRO2 of 16% were noted in the intact animals after naloxone administration. The CBF in MCA territory reduced to 24% after MCA occlusion and 85% in contralateral hemisphere. MCA occlusion had no significant effect on either mean BP or the difference in oxygen concentration between arterial and superior sagittal sinus blood. Naloxone increased these reduced CBF and mean BP significantly, but increase in oxygen consumption was not significant. There was no difference between early therapy group and delayed therapy group in pathological study. The effect of naloxone was temporary. Therapy of repeated or continuous naloxone administration appears to be effective during temporary focal cerebral ischemia.