Insulin exerts cardiovascular actions by stimulating nitric oxide (NO) release and sympathetic neural outflow. It is unclear, however, whether insulin stimulates muscle blood flow (and NO release) by a direct action at the vasculature and/or by stimulating neural vasodilator mechanisms. In these studies we used patients with regional sympathectomy to examine the vascular actions of insulin in the presence and absence of sympathetic vasoconstrictor and vasodilator innervation. A 2-hour insulin (6 pmol/kg per minute)/glucose clamp increased muscle blood flow in both innervated and denervated limbs by roughly 40% (P<0.01 versus baseline for both limbs). The vasodilation reached its maximum within the first 30 to 45 minutes of insulin/glucose infusion in sympathetically denervated limbs, but only at the end of the infusion in innervated limbs (P<0. 01, denervated versus innervated limb). Infusion of a NO synthase inhibitor (N(G)-monomethyl-L-arginine [L-NMMA]) increased baseline arterial pressure, abolished the vasodilation in the denervated limb, and led to a significant additional increase in arterial pressure during the clamp, but did not alter whole body glucose uptake. Our data indicate that insulin stimulates blood flow in sympathectomized limbs by a direct action at the vasculature. This effect is mediated by stimulation of NO release and appears to be masked by the sympathetic vasoconstrictor tone in innervated limbs.