The purpose of the present study was to investigate the degree to which peak heart rate is reduced during exhaustive exercise in acute hypoxia. Five sea-level lowlanders performed maximal exercise at normobaric normoxia and at three different levels of hypobaric hypoxia (barometric pressures of 518, 459, and 404 mmHg) in a hypobaric chamber and while breathing 9% O(2) in N(2). These conditions were equivalent to altitudes of 3300, 4300, 5300, and 6300 m above sea level, respectively. At 4300 m, maximal exercise was also repeated after 4 and 8 h. Peak heart rate (HR) decreased from 191 (182-202) (mean and range) at sea level to 189 (179-200), 182 (172-189), 175 (166-183), and 165 (162-169) in the acute hypoxic conditions. Peak HR did not decrease further after 4 and 8 h at 4300 m compared to the acute exposure at this altitude. Between barometric pressures of 518 and 355 mmHg (approximately 3300 and 6300 m), peak HR decreased linearly: peak HR(hypobaria) = peak HR(sea level) - 0.135 x [hypobaria(3100) - hypobaria (mmHg)]; or peak HR(altitude) = peak HR(sea level) - 0.15 x (altitude - 3100 m). This corresponds to approximately 1-beat x min(-1) reduction in peak HR for every 7-mmHg decrease in barometric pressure below 530 mmHg (approximately 130 m of altitude gained above 3100 m). At termination of exercise, maximal plasma lactate and norepinephrine concentrations were similar to those observed during maximal exercise in normobaric normoxia. This study clearly demonstrates a progressive decrease in peak HR with increasing altitude, despite evidence of similar exercise effort and unchanged sympathetic excitation.