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Nitric oxide synthase (NOS) inhibition has been shown in humans to attenuate exercise-induced increases in muscle glucose uptake. We examined the effect of infusing the NO precursor l-arginine (l-Arg) on glucose kinetics during exercise in humans. Nine endurance-trained males cycled for 120 min at 72 ± 1% V̇o_{2 peak} followed immediately by a 15-min “all-out” cycling performance bout. A [6,6-²H]glucose tracer was infused throughout exercise, and either saline alone (Control, CON) or saline containing l-Arg HCl (l-Arg, 30 g at 0.5 g/min) was coinfused in a double-blind, randomized order during the last 60 min of exercise. l-Arg augmented the increases in glucose rate of appearance, glucose rate of disappearance, and glucose clearance rate (l-Arg: 16.1 ± 1.8 ml·min⁻¹·kg⁻¹; CON: 11.9 ± 0.7 ml·min⁻¹·kg⁻¹ at 120 min, P < 0.05) during exercise, with a net effect of reducing plasma glucose concentration during …