Background: L-Arginine, the substrate of nitric oxide (NO) synthase, and NG-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of endothelial NO synthase, are used to analyze endothelial function of the renal vasculature. However, little is known about the appropriate dose of L-arginine to be used and the duration of action of L-arginine and L-NMMA. Methods: Twenty-nine healthy male subjects (age, 27 ± 1 years) were examined. In protocol 1 (N = 17), L-arginine at low (100 mg/kg) and high dose (250 mg/kg), and high-dose L-arginine combined either with L-NMMA (total dose, 4.25 mg/kg; N = 9) or placebo (N = 8) were given. In protocol 2 (N = 12), L-NMMA was given before L-arginine infusion (100 mg/kg). Glomerular filtration rate (GFR) and renal plasma flow (RPF) were measured at rest and at the end of each infusion step. Results: In protocol 1, L-arginine dose dependently increased RPF and GFR (RPF: 599 ± 19 v 630 ± 18 v 690 ± 24 mL/min, P < .05; GFR: 111 ± 3 v 115 ± 3 v 121 ± 3 mL/min, P < .01; for baseline, L-arginine 100 mg/kg and 250 mg/kg, respectively). However, these changes could not be antagonized by coinfusion of L-NMMA to L-arginine 250 mg/kg: RPF and GFR remained unchanged in both the placebo and the L-NMMA group. In protocol 2, L-NMMA decreased RPF (492 ± 18 v 567 ± 27 mL/min, P < .01) and increased GFR (122 ± 4 v 118 ± 3 mL/min, P < .05). These changes could only be partially reversed by subsequent infusion of L-arginine (RPF: 533 ± 15 mL/min; GFR: 121 ± 4 mL/min; both parameters P = NS v L-NMMA and v baseline). Conclusions: L-arginine at a dose of 100 mg/kg is sufficient to analyze endothelial function of the renal vasculature. The prolonged effect of L-NMMA and L-arginine must be taken into account in study protocols using both substances. Thus, stimulation and blockade of NO synthase cannot be examined in the same protocol.