Seven healthy young men were submitted twice to a hypoxia tolerance test at a simulated altitude (3000m). Their first acute exposure was in a hypobaric chamber; and the second, in a hypoxic tent. Cardiorespiratory parameters and heart rate variability measurements were obtained under each hypoxic condition. A significant decrease of 6% to 8% compared to normal oxygen conditions was observed in arterial oxygen saturation (SpO2) in both hypoxic conditions at rest; whereas exercise led to decreases of 10% in SpO2 despite an increase of 27% in respiratory minute volume. The low frequency (LF) and high frequency (HF) components of heart rate variability significantly changed from normoxia (LF: 37.1, HF: 62.9, LF/HF: 1.27) to hypobaric hypoxia (HH) (LF: 49.1, HF: 50.6, LF/HF: 1.96). However, these changes were not observed under normobaric hypoxia. Thus, heart rate variability behaved differently in the two hypoxic conditions, supporting the hypothesis that normobaric hypoxia and hypobaric hypoxia are not equal stimuli to the cardiovascular and respiratory systems. A correlation was found between sympathetic and vagal modulations in normoxia and SpO2 at exercise under hypobaric hypoxia (HH). Individuals with higher sympathetic modulation (LF%) in normoxia had higher SpO2 at exercise under HH (r=0.808, P<0.05) and individuals with higher vagal modulation (HF%) in normoxia showed a trend to lower SpO2 in exercise under HH (r=-0.636, P=0.125). This opens up the possibility of using this correlation as a tool for predicting the individual capacity to altitude acclimatization. © 2011 Consell Català de l'Esport. Generalitat de Catalunya.