The amino acid L-arginine, the precursor of nitric oxide (NO) synthesis, induces vasodilation in vivo, but the mechanism behind this effect is unclear. There is, however, some evidence to assume that the L-arginine membrane transport capacity is dependent on insulin plasma levels. We hypothesized that vasodilator effects of L-arginine may be dependent on insulin plasma levels. Accordingly, we performed two randomized, double-blind crossover studies in healthy male subjects. In protocol 1 (n = 15), subjects received an infusion of insulin (6 mU · kg¹ · min¹ for 120 min) or placebo and, during the last 30 min, L-arginine or D-arginine (1 g/min for 30 min). In protocol 2 (n = 8), subjects received L-arginine in stepwise increasing doses in the presence (1.5 mU · kg¹ · min¹) or absence of insulin. Renal plasma flow and glomerular filtration rate were assessed by the para-aminohippurate and inulin plasma clearance methods, respectively. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation, and mean flow velocity in the ophthalmic artery was measured with Doppler sonography. L-arginine, but not D-arginine, significantly increased renal and ocular hemodynamic parameters. Coinfusion of L-arginine with insulin caused a dose-dependent leftward shift of the vasodilator effect of L-arginine. This stereospecific renal and ocular vasodilator potency of L-arginine is enhanced by insulin, which may result from facilitated L-arginine membrane transport, enhanced intracellular NO formation, or increased NO bioavailability.