The purpose of the study was to investigate a prolonged effect of hypoxia on the reproductive function and development of pups of white mice. Beginning with the age of 2 months the animals were exposed to an altitude of 5500 m for 6 hours a day during 9 months. The mice that were so adapted to and mated in hypoxia did not deliver any offspring. The animals that were adapted to hypoxia during intrauterine and postnatal development and mated at 5500 m delivered normal although much smaller litters.