The examinations were carried out during the 27th Soviet Antarctic expedition. Baseline data were collected before the departure of the test subjects to the Antarctic Region. Prior to their ascent to the high mountain area they were divided into two groups with a high and a low level of hypoxic tolerance in terms of the work capacity index calculated on the basis of standard bicycle ergometry tests. Heart rate, body temperature and salivary content of sodium and potassium were measured 6 times a day at 4-hour intervals. The results obtained were treated by nonparametric tests. It was found that on adaptation day 30 the subjects with low hypoxic tolerance and nonspecific resistance developed changes in biorhythm amplitudes and phases and showed ultradian components with a 12-hour period. By contrast, the subjects with high hypoxic tolerance retained the ability to maintain circadian patterns. By the middle of the wintering time the circadian rhythms shifted towards ultradian components regardless of individual hypoxic tolerance.