This study was performed to investigate the placental transfer and the human fetal utilization of amino acids at term of pregnancy. The plasma levels of 23 free amino acids in both the umbilical circulation ( umbilical vein and artery ) and the uterine circulation ( uterine vein and iliac artery ) of 34 pregnant women were measured at delivery by the cesarean section. In the umbilical circulation, 9 amino acids ( alanine, lysine, valine, leucine, arginine, isoleucine, ornithine, cystine, ${alpha}-aminobutyrate$ ) were significantly higher and 2 amino acids ( glutamate, aspartate ) were significantly lower in the umbilical vein than in the umbilical artery. In the uterine circulation, alanine, tyrosine and methionine were significantly lower in the uterine vein than in the iliac artery. Glutamate was significantly lower in the uterine vein than in the iliac artery. According to these results, the origin of fetal plasma amino acids was discussed in terms of the metabolic conversions which would occur in the Placenta and the fetal utilization of amino acids was estimated.