In mice of different ages from the OF1 mouse strain, males are less resistant than females to a normobaric hypoxia obtained in a few hours by a progressive lowering of normoxic PO2 with nitrogen flushing. Injection of estradiol to castrated males and spayed females increases hypoxic survival. Neonates which have been injected with a high dose of estradiol show, when adult, a high hypoxic resistance. In adult females, hypoxic survival is lower during diestrus than during estrus. Pregnancy decreases resistance to hypoxia. Experiments, performed with males and females of different ages, show the effects of sex-related dimorphism and aggressiveness. Hypoxias at various ambient temperatures demonstrate that the sex difference in hypoxic survival persists in spite of variations in rectal temperatures.