The objective of the presented studies is to attempt an evaluation of heart rate (HR) and heart rate variability (HRV) regulatory mechanisms in the presence of autonomous nervous system (ANS) components in transient exposure to high altitude hypoxia. During 24 hrs including a stay in hypobaria, the participants had their HR continuously recorded using the Holter method. The following parameters were calculated at rest and during the stay in a thermobarochamber: spectral power in low frequency bands (LF) 0.04-0. 15 Hz and high frequency bands (HF) 0.15-0.5 Hz, and the sympathetic-parasympathetic balance index LF/HF. Under hypobaric conditions, a decrease in mean spectral power of R-R intervals was noted within both frequency ranges, compared with the study performed in normobaria. The observed differences were larger at daytime.