The resistance to acute hypoxia in male Wistar rats was evaluated by the period of survival after exposure to high-altitude hypoxia (11.5 km above see level). The study was performed during daytime (13.00-21.00) in autumn. The fatal rat population was characterized by the log-normal distribution of survival periods. The rats with low and moderate resistance to hypoxia exhibited similar diurnal variations in it with gradual decrease by the end of daytime more pronounced in low-resistant rats. The rats with high resistance showed relatively constant resistance to hypoxia which decreased only at 21:00. All groups revealed a relatively stable resistance to hypoxia from 16:00 to 18:00. These variations in the resistance to hypoxia should be taken into consideration when planning experimental research. ©1999 Kluwer Academic/Plenum Publishers.