Swiss albino mice were maintained in closed decompression chambers: 1) continuously at simulated altitudes of 14,200 and 18,000 ft; 2) intermittently, 6 hr/day, at 20,000, 21,500, and 25,000 ft; 3) continuously at sea level with atmosphere of 12% oxygen, 88% nitrogen. All animals were acclimatized before exposure. In all cases mating behavior was normal, and there was no functional impairment of male fertility. Impregnation was observed with all mated, adapted females in almost all experiments. Exceptions were considered due to early resorption of embryos. No significant effect of hypoxia was noted on implantation, placentas, vascularization, gestation time, or parturition. Adverse effects of hypoxia were manifested primarily in resorption of fetuses when they had attained a critical size (about 7 mm). The incidence of resorption correlated directly with stress level, expressed as duration and intensity of hypoxia, and is considered to be contingent on the critically respiring mass of fetal tissue.