Endogenous nitric oxide (NO) is an important mediator of vasodilatation, bronchodilatation and lung inflammation. We hypothesised that the exhaled NO level may be modified in some endurance-trained athletes during and after intense exercise. Nine athletes with exercise-induced hypoxaemia (EIH), 12 athletes without EIH and 10 untrained subjects exercised for 15 min at 90% maximal oxygen consumption (VO(2)max). Exhaled NO was measured during exercise, and after 1 h and 22 h of recovery. Exhaled NO concentration ( C(NO)) decreased significantly during exercise in all subjects and returned to basal values after 1 h of recovery with no further modification. Exhaled NO output (V(NO)) rose significantly during exercise, rapidly dropped down following exercise and was similar to resting values after 1 h and 22 h of recovery. The results also showed that C(NO) and V(NO) were significantly lower in the athletes with EIH in comparison with the untrained subjects (V(NO) was 5.32 +/- 0.77 nmol/min versus 3.61 +/- 0.72 nmol/min at rest, 18.52 +/- 1.50 nmol/min versus 15.00 +/- 2.06 nmol/min during heavy exercise, and 5.52 +/- 1.04 nmol/min versus 3.79 +/- 0.76 nmol/min after 22 h recovery, in untrained subjects and EIH athletes, respectively). These findings do not confirm the hypothesis of pulmonary inflammation associated with EIH. However, potential NO epithelial down-regulation may occur and contribute to the development of gas exchange abnormality in some endurance-trained athletes.