Nitric oxide is an important regulator of the regional cerebral vascular tone. We compared the magnitude of nitric oxide-related changes in the vascular tone by studying the regional cerebral blood flow (rCBF) and vascular resistance in conscious and isoflurane-anesthetized rats by using a nitric oxide synthase in-hibitor, NG-nitro-L-arginine methyl ester (L-NAME). In the conscious group (n = 12), after cannulation of a femoral artery and two veins under isoflurane anesthesia, rats were allowed to remain awake for 90 min. In the anesthetized group (n = 18), rats were anesthetized with 2% isoflurane and mechanically ventilated. Six rats in each group were treated with L-NAME (2 mg·kg−1·min−1 for 30 min) or saline. For the remaining rats in the isoflurane-anesthetized group (n = 6), arterial blood pressure was increased by phenylephrine infusion to the same level as that in the L-NAME-treated, isoflurane-anesthetized rats. Regional vascular resistance was determined by the ratio of mean arterial blood pressure and rCBF which was measured by [14C]iodoantipyrine. L-NAME significantly increased mean arterial blood pressure in both the conscious (123 to 158 mm Hg) and anesthetized (82 to 144 mm Hg) rats. Regional vascular resistance increased significantly in all 12 brain regions studied with the average value increasing from 1.19 ± 0.33 mm Hg·mL−1·min·100 g to 2.22 ± 0.48 (P < 0.0001) in the conscious and from 0.78 ± 0.27 to 1.61 ± 0.48 (P < 0.0001) in the isoflurane-anesthetized rats. L-NAME significantly decreased rCBF in the conscious animals. In the isoflurane-anesthetized rats, L-NAME significantly decreased CBF only in the medulla and pons, and phenylephrine infusion significantly increased rCBF in most of the brain regions. Our study suggests that nitric oxide plays an important role in regulating cerebral vascular tone in conscious as well as in isoflurane-anesthetized rats.