Nitric oxide (NO) contributes to the regulation of regional blood flow. Inhibition of NO synthesis increases blood pressure and vascular resistance. Using radioactive microspheres and the substrate antagonist N omega-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg) to block NO synthesis, we tested the hypothesis that there is a significant interaction between the vasodilator NO and the vasoconstrictor angiotensin II, which regulates regional hemodynamics. Further, we investigated the influence of anesthesia on this interaction. L-NAME increased blood pressure, decreased cardiac output, and increased total peripheral resistance in both anesthetized and conscious rats. In anesthetized rats, L-NAME decreased blood flow to visceral organs (i.e. kidney, intestine, and lung) but had little effect on blood flow to the brain, heart, or hindlimb. Treating anesthetized rats with the angiotensin II receptor antagonist losartan (10 mg/kg) attenuated the decrease in cardiac output and the increase in total peripheral resistance without affecting the pressor response to L-NAME. Losartan also attenuated the visceral hemodynamic responses to L-NAME. In conscious rats, L-NAME decreased blood flow to all organ beds. Treating these rats with losartan only marginally attenuated the increase in total peripheral resistance to L-NAME without significantly affecting the pressor response or the decrease in cardiac output. Losartan had no effect on the regional hemodynamic responses to L-NAME. These data suggest that NO-mediated vascular relaxation is an important regulator of total peripheral and organ vascular resistance. (ABSTRACT TRUNCATED AT 250 WORDS)