1. Pharmacological stimulation of the synthesis of nitric oxide (NO) may be important in the prevention or treatment of cardiovascular diseases. 2. There is much discussion as to whether the precursor of NO, L-arginine, is able to stimulate basal endothelial NO production. L-Arginine is known to have vasodilating effects. However, it is not clear whether L-arginine-induced vasodilatation is attributable to an increase in NO production or to other systemic effects of L-arginine. 3. To investigate further the mechanisms of the L-arginine-induced vasodilatation, we compared the responses to L-arginine with those to saline and L-lysine in healthy subjects. L-Lysine is not a substrate for NO synthesis, but shares many of L-arginine's other properties. 4. During L-arginine infusion, blood pressure decreased [systolic blood pressure from 120.2 (SD 8.8) to 117.3 (12.1) mmHg (P = 0.05); diastolic blood pressure from 65.3 (5.9) to 61.6 (7.9) mmHg (P < 0.01)], and heart rate and extracellular fluid volume increased. The total peripheral vascular resistance decreased during L-arginine infusion by 18.0 (11.4)% (P < or = 0.05 compared with baseline and compared with L-lysine infusion). These results indicate vasodilation. No changes were observed during L-lysine and saline infusion. 5. Plasma cyclic GMP (the second messenger for NO) increased during L-arginine but also during L-lysine infusion [from 5.7 (1.2) to 6.8 (1.7) nmol/l (P < 0.01), and from 5.8 (1.8) to 7.0 (2.9) nmol/l (P < 0.05) respectively]. Plasma L-citrulline (a by-product of NO synthesis from L-arginine) increased during L-arginine infusion from 30.6 (7.5) to 47.1 (9.9) mumol/l (P < 0.001), but also during L-lysine infusion from 32.7 (6.5) to 42.0 (8.3) mumol/l (P < 0.001). 6. Plasma electrolytes and atrial natriuretic peptide concentrations responded similarly to L-arginine and L-lysine infusion, indicating similar effects on osmolality, plasma volume expansion and potassium distribution. 7. In conclusion, although L-lysine infusion had effects that were similar to those of L-arginine infusion, no vasodilatation was observed. Therefore, these effects cannot account for the L-arginine-induced vasodilatation. This finding indirectly supports the hypothesis that the vasodilatation during L-arginine infusion might be mediated by an increase in NO synthesis. If so, our data suggest that the presumed markers for NO synthesis, plasma cyclic GMP and L-citrulline concentrations, do not accurately reflect this increase. Instead, the rise in plasma cyclic GMP may be related to the rise in ANP. The rise in L-citrulline may be related to competition with L-arginine for the same cell membrane transport mechanism and to stimulation of the urea cycle.