Plasma proteins tagged in vivo by feeding D-L-lysine-epsilon-C(14) to donor dogs have been administered to pregnant dogs by both oral and intravenous routes. A relatively small percentage of the C(14) activity originally incorporated in these proteins is found to pass from mother to fetus after intravenous injection. The amount transferred tends to increase with the length of gestation period and total number of fetuses. Plasma protein labeled with I(131) does not cross the placenta in the dog, but does in the rabbit. Evans blue dye does not cross the placenta of the dog. After oral administration of labeled plasma protein or lysine, C(14) is transferred promptly and in considerable quantity to the fetus. Labeled plasma proteins disappear more rapidly from the circulation of pregnant than of normal dogs. This increased metabolic turnover occurs without excretion of any excess waste metabolites. The chorionic epithelium, gram for gram, is probably 2 to 3 times as active as the hepatic epithelium in protein metabolism. These findings indicate an important placental function related to maternal and fetal protein metabolism. While the placenta utilizes maternal plasma proteins and amino acids, in a quantitative sense the latter appear to supply the major nitrogen needs of the growing fetus.