Arbino acids are actively transported across the mammalian placenta, crncentrations in fetal blood being higher than those in the maternal circu-Elevated concentrations of phenylalanine were induced by dietarv means blood of pregnant rhesus monkeys, and the active transport mechanism vident at both normal and elevated concentrations. A normal placental may thus magnify a maternal biochemical abnormality and produce a profound disturbance in the fetus.