PURPOSE: To determine if heart rate variability, body composition and hypoxia exposure time could help to identify risk of hypoxia in a group of COLAF aircrew that train in hypobaric Chamber at the Aerospace Medical Center. METHODS: Previous ethics committee approval and signing of the informed consent, 60 pilots between 20 and 40 years were selected. Body composition was obtained by impedance with a Tanita ® bascule TBA-300A (weight, height, BMI, muscle weight, fat percentage and water percentage). The volunteers attended to a hypoxia physiological training where they were exposed to profile IV-A COLAF (30 minutes denitrogenation with 100% oxygen (DNT), climb to 25,000 feet with exposure to hypoxia and descent to ground level). During this profile heart rate was monitored through the RR record with a Polar ® heart rate monitor RS800 and the minimum percentage of oxygen saturation in hypoxia exercise previous recovery was recorded. Hypoxia permanence time (TH) and power frequency response ANS Low Frequency (LF), High Frequency (HF) and LF/ HF were analyzed. For LF and HF, modeling by Fast Fourier Transform (FFT) was implemented. Lilliefors test to verify normality was applied. For the relationship between variables with non-normal distribution Chi square test was applied. And for normal distribution direct correlation was applied. RESULTS: Direct correlation (p-value <0.05) between TH (4.24 ± 1.74 min) and water percentage (44.46 ± 6.11) was observed. Additionally, a significant difference in LF / H was found among the three moments of the flight profile using the Wilcoxon test was observed in 21 pilots due to loss of signal in the other 39. While in HF there were only significant between moments: DNT - Hypoxia and Hypoxia - descent to ground level. CONCLUSIONS: Pilots with highest percentage of water and muscular mass tolerated longer exposure to hypoxia at 25000 feet. Also, changes in oxygen partial pressure determine changes on autonomic nervous system, thus hypoxia exposure increase sympathetic modulation over parasympathetic that could be modify by body composition.