Increasing ventilation (VE) during hypoxic exercise may help to defend arterial O2 saturation (SaO2) and VO2max however, many athletes experience limitations to ventilatory flow and are not able to increase VE at high workrates. Five of 19 highly trained endurance athletes screened had < 5% of their tidal flow volume loop during maximal exercise meet the boundary set by their maximal resting flow volume loop. These five athletes were grouped as non-flow limited and compared to the five athletes who demonstrated the greatest percent of tidal volume flow limitation (56 +/- 11%) during maximal exercise (flow limited). Each athlete completed two incremental treadmill tests to exhaustion: normoxia and hypoxia (FI(O2) = 0.187). Non-flow limited athletes increased VE at VO2max from normoxia to hypoxia (140.9 +/- 13.4 vs. 154.7 +/- 11.9 L/min, P < 0.05), while flow limited athletes did not (159.5 +/- 9.4 vs. 162.3 +/- 6.0 L/min). The decline in SaO2 at VO2max from normoxia to hypoxia was not significantly different between groups. We conclude that athletes with little or no expiratory flow limitation are able to increase VE during maximal exercise in mild hypoxia, compared to athletes with significantly higher degrees of mechanical limitation. However this 'mechanical ventilatory reserve' does not appear to influence the ability to defend SaO2 or VO2max during maximal exercise in mild hypoxia.