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The plasma levels of 20 free amino acids in the umbilical veins and umbilical arteries of 8 premature (29–36 weeks gestation) and 16 mature (38–42 weeks gestation) newborn infants were measured at delivery. In premature newborn infants, most of the 20 amino acids were significantly higher in the umbilical vein than in the umbilical artery. Only glutamic acid was significantly lower in the umbilical vein than in the umbilical artery. In mature newborn infants, 7 (Ala, Lys, Leu, Val, He, Phe and His) of the 20 plasma amino acids were significantly higher and 4 (Glu, Gly, Ser and Om) were significantly lower in the umbilical vein than in the umbilical artery. These results indicate that the relative contribution of individual amino acids to the placental supply of nitrogen to the human fetus discernibly changes with increasing fetal age during the last trimester of gestation.