Twelve trained runners, who were moderate-altitude (1,520 m) residents, were acutely exposed to sea level and 3,050-m altitude in a hypo-hyperbaric chamber. At 1,520 m, maximum O2 consumption (VO2 max) was 67.7 +/- 0.9 mL/kg/min, minute ventilation (V) at maximum exercise was 169.8 +/- 4.6 L/min, and arterial O2 saturation (SaO2) decreased from 93.3% +/- 0.9% at rest, to 84.5% +/- 1.6% at maximum exercise. Exposure to 3,050 m produced a lower VO2max (56.5 +/- 1.5, -16.5%), a similar V at maximum exercise (169.9 +/- 6.0), and a similar fall in SaO2 (from 89.1% +/- 0.8% to 79.5% +/- 0.8%). Exposure to sea level increased VO2max to 72.4 +/- 1.4 (+6.9%), reduced V at maximum exercise to 158.8 +/- 6.5, and induced a smaller fall in SaO2 (from 96.9% +/- 0.4% to 92.1% +/- 1.0%). These changes are comparable with those reported previously in athletes at sea level exposed to similar altitudes, suggesting that residence at 1,520 m does not improve VO2max in highly trained athletes acutely exposed to lower or higher altitudes.