BACKGROUND: Nitric oxide (NO) regulates vascular tone and blood pressure, and studies in animals suggest that it does so, at least in part, by modulating sympathetic neural outflow. Loss of NO-induced vasodilator tone and restraint on sympathetic vasoconstrictor outflow could lead to exaggerated vasoconstrictor and pressor responses to physical stress in humans. METHODS AND RESULTS: To determine the role of NO in the modulation of central sympathetic outflow and vascular tone at rest and during a physical stress, we tested effects of systemic inhibition of NO synthase by N(G)-monomethyl-L-arginine (L-NMMA) infusion (a stereospecific inhibitor of NO synthase) on sympathetic nerve activity (microneurography), regional vascular resistance, and blood pressure at rest and during static handgrip. The major new findings are that (1) under resting conditions, L-NMMA infusion, which increased mean arterial pressure by approximately 10%, did not have any detectable effect on muscle sympathetic nerve activity, whereas a similar increase in arterial pressure evoked by phenylephrine infusion (an NO-independent vasoconstrictor) decreased the rate of sympathetic nerve firing by approximately 50%; (2) during static handgrip, the exercise-induced sympathetic nerve responses were preserved during L-NMMA infusion but markedly attenuated during phenylephrine infusion; and (3) the L-NMMA-induced loss of vasodilator tone did not result in exaggerated exercise-induced pressor and calf vasoconstrictor responses. CONCLUSIONS: These findings indicate that NO is involved in the central regulation of sympathetic outflow in humans and suggest that both neuronal and endothelial NO synthesis may contribute to the regulation of vasomotor tone.