The role of nitric oxide in the regulation of cerebrocortical microvascular tone and autoregulation of cerebral blood flow (CBF) was examined in 24 anesthetized cats. The local cerebral blood volume (CBV), mean transit time of blood (MTT), and CBF in the cortex were measured by our photoelectric method. CBV represents the cumulative dimensions of the cerebral microvessels. Intravenous injection of 0.35-0.7 mg/kg/min NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthesis, significantly increased mean arterial blood pressure (MABP; 8.4-14.1%, P < 0.01), decreased CBV (15.2-28.7%, P < 0.01), and decreased CBF (20.0-29.8%, P < 0.01) in a dose-related manner. The changes in MABP, CBV, and CBF elicited by L-NMMA were inhibited (P < 0.05) by simultaneous infusion of 35 mg/kg/min L-arginine. Autoregulation of CBF was examined during controlled hypotension of -30 to -40 mmHg (artificial bleeding) and recovery of blood pressure (reinfusion of blood). Although CBF remained constant with blood pressure changes in the control state (delta CBF/delta MABP of 0.037 +/- 0.155 with hypotension), CBF became dependent on blood pressure changes (delta CBF/delta MABP of 0.478 +/- 0.135, P < 0.05) during infusion of 0.35 mg/kg/min L-NMMA. It is concluded that nitric oxide participates in both the regulation of basal tone of cerebral microvessels and the autoregulation of CBF.