Blood flow responses to alterations in mean arterial blood pressure (MABP) were measured in the cerebral cortex, subcortex, midbrain, and spinal cord of awake rats. Data were compared with those of rats anesthetized with an i.v. fentanyl infusion and inspired nitrous oxide (N2O). Regional cerebral blood flow was measured using radioactive microspheres in the following blood pressure ranges: (a) <40 mm Hg; (b) 40-60; (c) 60-80; (d) 80-100; (e) 100-120; (f) 120-140; (g) 140-160; and (h) >160. Blood pressure was increased with phenylephrine or decreased with trimethaphan combined with blood withdrawal. Cerebral blood flow was not measured when MABP was less than 60 mm Hg in awake rats. Autoregulation was seen in all brain areas between 60 and 140 mm Hg in both treatment groups. Although regional cerebral blood flow was not different between the two treatment groups, PaCO2 was 2-4 mm Hg lower in awake rats. This suggests that PaCO2-corrected cerebral blood flow may be 10-20% lower with fentanyl-N2O anesthesia.