Insulin exerts effects on the vasculature that (a) may play a role in the regulation of blood pressure; and (b) by boosting its own delivery to target tissues, also have been proposed to play an integral part in its main action, the promotion of glucose disposal. To study the role of nitric oxide (NO) in the mediation of insulin's effects on the peripheral vasculature, NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of the synthesis of endothelium-derived NO, was infused into the brachial arteries of healthy volunteers both before, and at the end of a 2-h hyperinsulinemic (6 pmol/kg per min) euglycemic clamp. L-NMMA (but not norepinephrine, an NO-independent vasoconstrictor) caused larger reductions in forearm blood flow during hyperinsulinemia than at baseline. Moreover, L-NMMA prevented insulin-induced vasodilation throughout the clamp. Prevention of vasodilation by L-NMMA led to significant increases in arterial pressure during insulin/glucose infusion but did not alter glucose uptake. These findings indicate that insulin's vasodilatory effects are mediated by stimulation of NO release, and that they play a role in the regulation of arterial pressure during physiologic hyperinsulinemia. Abnormalities in insulin-induced NO release could contribute to altered vascular function and hypertension in insulin-resistant states.