In order to systematically assess the effects of acute exposure to moderate hypoxia on aerobic capacity (VO2max), 12 men (regular participants in recreational distance running) performed six treadmill-graded exercise tests (GXTs) in a hypobaric chamber. GXTs 1 and 6 were performed at ambient (control) altitude (362 m, barometric pressure = 730 mmHg). GXTs 2-5 were administered during 1-2 h of exposure to barometric pressures of 681, 656, 632, and 574 mmHg simulating altitudes of 914, 1219, 1524, and 2286 m, respectively, with the order of presentation randomized and blinded for each subject. The mean VO2max for GXTs 1 and 6 (control altitude) were essentially identical with a test-retest correlation of r = 0.92. During peak exercise, HR max was unchanged by hypoxia, while VO2max was significantly lower than the control by 4,8, 6.9, and 11.9% at 1219, 1524, and 2286 m, respectively. SaO2@max percent during maximal exercise was significantly reduced from the control by 3.5, 3.6, 7.0, and 11.6% at 914, 1219, 1524, and 2286 m, respectively. It was concluded that VO2max, in physically well-conditioned persons living at 362 m, is reduced during acute exposure to 1219 m and above.