The influence of nitric oxide (NO) on basal vascular tone varies with different hypertensive models or vascular beds. The goal of the present study was to examine the role of NO in the maintenance of resting cerebral blood flow (CBF) during chronic hypertension. In 9-10 months old Wistar-Kyoto (WKY) rats (n=47) and spontaneously hypertensive rats (SHR;n=47) anesthetized with pentobarbital sodium (60 mg/kg i.p.), regional CBF of the right parietal cortex was monitored by laser-Doppler flowmetry. Reductions in CBF in response to intravenous infusion of the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 1, 3, 10, and 30 mg/kg) were similar between WKY rats (17 +/- 6 approximately 43 +/- 6%; means +/- SE) and SHR (15 +/- 6 approximately 48 +/- 6%) while arterial blood pressure was maintained on the baseline level by controlled hemorrhage. Effects of L-NAME (3 mg/kg i.v.) on arterial blood pressure and CBF were almost completely inhibited by L-arginine (300 mg/kg i.v.), but not by D-arginine (300 mg/kg i.v.). In addition, intravenous infusion of L-arginine (300 mg/kg) alone did not affect resting CBF in both WKY rats and SHR. Thus, these findings suggest that 1) NO plays an important role in the maintenance of resting CBF in both normotensive and chronically hypertensive rats and 2) the contribution of NO to the maintenance of resting CBF is not altered during chronic hypertension.