It has been hypothesized that one reason for decreased .VO(2max) in hypoxia could be the lower maximal exercise intensity achieved in incremental, time or distance trial tests. We hypothesized that (1).VO(2max) would be decreased at altitude even when exercising at the same absolute maximal exercise intensity as at sea level and; (2) the decline in .VO(2max) in endurance-trained athletes (ETA) would be linear across the range from sea level through moderate altitudes. Eight ETA performed combined .VO(2max) and performance tests running to exhaustion at the same speed in a randomized double blind fashion at simulated altitudes of 300, 800, 1,300, 1,800, 2,300 and 2,800 m above sea level using a hypobaric chamber. Douglas bag system was used for respiratory measurements and pulse oximetry was used to estimate arterial O(2) saturation. .VO(2max) declined linearly from 66+/-1.6 ml kg(-1) min(-1) at 300 m to 55+/-1.6 ml kg(-1) min(-1) at 2,800 m corresponding to a 6.3% decrease per 1,000 m increasing altitude (range 4.6-7.5%). Time to exhaustion (performance) at a constant velocity associated with 107% of sea level .VO(2max) decreased with 14.5% (P<0.001) per 1,000 m altitude between 300 and 2,800 m. Both .VO(2max) and performance decreased from 300 to 800 m (P<0.01; P<0.05). Arterial haemoglobin oxygen saturation at test cessation (SpO(2min)) declined from 89.0+/-2.9% at 300 m to 76.5+/-4.0% at 2,800 m (P=0.001). This study report that in ETA during acute exposure to altitude both performance and .VO(2max) decline from 300 to 800 m above sea level and continued to decrease linearly to 2,800 m.