THAT anoxia at critical stages of prenatal development is an important cause of acquired malformations of the fetus is an inference derived from earlier studies of mongolism, retrolental fibroplasia and tracheoesophageal fistula.1This hypothesis has been tested experimentally by submitting pregnant mice at various stages of gestation to rarefied atmospheres. MATERIALS AND METHODS The white mouse is a prolific, easily handled mammal in which the onset of pregnancy can be determined with much certainty by means of an inspissated plug appearing in the vagina shortly after copulation. Females used were chosen at random from a laboratory strain maintained for the past four years with avoidance of brother and sister mating. Fertility has remained satisfactory, with a mean of 8.2 normal pups per litter. For the purposes of these experiments the day on which the copulation plug appears is regarded as the first day of pregnancy, and it is our