The systemic and regional hemodynamic effects of inhibition of endothelium-derived relaxing factor/nitric oxide (EDRF/NO) were studied in awake, indomethacin-treated rats. The radiolabeled microsphere method was used to determine the cardiac output, systemic vascular resistance (SVR), and regional blood flows and regional vascular resistances in 12 tissues before and after infusion of the EDRF/NO synthesis inhibitor, NG-monomethyl-L-arginine (NMMA, 100 mg/kg), and after reversal of NMMA by infusion of L-arginine (300 mg/kg). NMMA infusion resulted in increases in the blood pressure and SVR. After NMMA, blood flows were decreased to the cerebrum, heart, kidney, spleen, gastrointestinal tract, skin, ear, and white fat, whereas flow in the hepatic artery was increased. Vascular resistances were increased in every tissue studied except the hepatic artery, in which the resistance decreased after NMMA. L-arginine restored the vascular resistance to control values in 8 of the 12 tissues. The magnitude of the increase in the regional resistance was not uniform among the organs studied, and ranged from a maximum of 253% in brown fat to 22% in heart. These results indicate that EDRF/NO is an important mediator of regional hemodynamic control in numerous tissues of the intact rat. The marked heterogeneity in the magnitude of basal EDRF/NO-dependent tone suggests that the mechanisms mediating this cardiovascular control system are regulated locally.