L-Arginine, the precursor of endothelium-derived relaxing factor (EDRF)/nitric oxide (NO), was administered intravenously in five patients with essential hypertension, one with renovascular hypertension, one with primary aldosteronism, and one with Cushing's syndrome. During the administration, the mean arterial pressure decreased concomitantly with an elevation of cardiac output and a fall in total peripheral resistance in all cases. Indicators of NO release in vivo such as plasma concentrations of L-citrulline and urinary excretion of nitrite/nitrate increased simultaneously during the administration. These results suggest that exogenous L-arginine can produce a vasodilatory effect via stimulating NO release in hypertensives.