We hypothesised that the effective inhibition of nitric oxide synthase (NOS), achieved via systemic infusion of NG-nitro-L-arginine methyl ester (L-NAME), would reduce the gas exchange threshold (GET) and the maximal oxygen uptake (VO2 max) during incremental cycle exercise in man if NO is important in the regulation of muscle vasodilatation. Seven healthy males, aged 18-34 years, volunteered to participate in this ethically-approved study. On two occasions, the subjects completed an incremental exercise test to exhaustion on an electrically-braked cycle ergometer following the infusion of either L-NAME (4 mk.kg-1 in 50 ml saline) or placebo (50 ml saline). At rest, the infusion of L-NAME resulted in a significant increase in mean arterial pressure (MAP; mean ± S.D. CON: 89 ± 8 vs. L-NAME: 103 ± 11 mmHg; P<0.05) and a significant reduction in heart rate (HR; CON: 60 ± 12 vs. L-NAME: 51 ± 8 b.min-1; P<0.01). At sub-maximal work rates, there was no significant difference in VO2 between the conditions and no difference in the GET (CON: 1.94 ± 0.47 vs. L-NAME: 2.01 ± 0.41 L.min-1). However, at higher work rates, differences in VO2 between the conditions became more pronounced such that VO2 max was significantly lower with L-NAME (CON: 4.02 ± 0.41 vs. L-NAME: 3.80 ± 0.34 L.min-1; P<0.05). The reduction in VO2 max was associated with a reduction in HR max (CON: 186 ± 10 vs. L-NAME: 178 ± 7 b.min-1; P<0.01). These results demonstrate that NOS inhibition with L-NAME has no effect on the GET but reduces VO2 max during large muscle group exercise in man, presumably by direct or indirect effects on cardiac output and muscle blood flow.