Resistance to acute hypoxia was evaluated by the life span after elevation to a simulated altitude of 11,500 m at 13.00-21.00 in different seasons of the same year. The period of investigation was divided into 3 parts: beginning (13.00-15.00), middle (16.00-18.00), and end (19.00-21.00) of the day. Animal life span decreased by the end of the day (the decrease was more pronounced in summer and autumn) and by the end of the year, i. e. in autumn (more pronounced in the middle and end of the day). The life span was longer in the middle of the day in spring in low- and medium-resistant rats and by the end of the year in medium-resistant rats (at the beginning of the day). Fluctuations in the life span during the day and year were more expressed in low- and medium-resistant rats. Differences in the life span of highly and low-resistant rats were the most pronounced in winter.