The relative importance of the prenatal and the postnatal transmission of passive immunity from actively immune mother rats to their young has been studied. It has been shown that the concentration of maternal antibodies in the circulating blood of the young rises rapidly from the 17th day of gestation to the 2nd day of lactation. Thereafter it remains relatively constant throughout lactation at levels approximately equal to those of the mother. It is evident that by far the greater part of the transmission of passive immunity occurs after birth, although there is transmission of a significant though small part before birth. Antibodies also occur in the amniotic and exocoelomic fluids and in the stomach contents of foetal rats.