Over 400 female rats were reared to sexual maturity on a diet deficient in vitamin A, supplemented with small amounts of carotene. The supplements were then stopped and the animals were mated. The first day of pregnancy was counted as the day on which spermatozoa were detected in the vaginal smear. Some of the rats were given no further supplement, but others received single large doses of vitamin A on some day between the 10th and 15th day of pregnancy. The incidence of congenital abnormalities was studied in the offspring of 261 rats in which pregnancy continued long enough for the examination to be made. Some lesions, such as ocular defects, diaphragmatic hernia and renal anomalies, were prevented by giving vitamin A on the 10th or 11th days, and the vitamin given later reduced the frequency of their occurrence, but the incidence of cardiac and lung abnormalities was increased by late dosing with vitamin A to a percentage considerably above that found in the young from mothers not so dosed. Presumably the administration of the vitamin kept alive foetuses which otherwise would have died before they became available for examination. It is concluded that vitamin A deficiency causes malformations during. the period of active organ formation, and not while the primordia are still undifferentiated. T. Moore.