High-intensity training may be difficult to sustain due to limitations in systemic oxygen transport, particularly at high altitudes. The purpose of this study was to examine the effects of a high-intensity training protocol using hyperoxic gas breathing in athletes "maximally trained" at an altitude of 1,600 m. Five subjects underwent progressive cycle training until they reached a plateau of aerobic capacity, maximal workload, and endurance time at 85 percent maximal workload. Significant decreases (2 to 6 percent) in arterial oxygen saturation were found after the 85 percent maximal workload tests. Training intensity was then increased to 95 percent maximal workload while the subjects breathed a gas mixture containing at least 70 percent oxygen. After 6 weeks of hyperoxic training, exercise parameters were compared with the plateau values obtained during the baseline training period. Total time during maximal cycle testing increased from 19.1 to 19.6 min (p = 0.015), heart rate at 85 percent maximal workload decreased from 168 to 163 bpm (p = 0.047), and endurance time at 85 percent maximal workload increased from 6.2 to 8.2 min (p = 0.012). There was a trend toward improvement of maximal workload. We conclude that hyperoxic training increases work capacity after attainment of "maximal training" at moderate altitude.