The free amino acids of cord blood as compared with maternal blood taken within a few minutes were studied by the use of ion-exchange column chromatography. Twenty-two paired plasma samples from immature, premature, and full-term caesarean sections and vaginal deliveries were studied. The material also included three cases of advanced maternal diabetes. In addition bloods from four healthy women after parturition were also analyzed to compare with blood obtained at the end of pregnancy. The results indicate that (a) all the ratios of free amino acids of cord to maternal blood are above 1 and mostly about 2, (b) the discrepancy is brought about by decreased maternal and increased fetal values, (c) the ratios obtained from blood taken at the second trimester of pregnancy are somewhat higher than those of full-term, (d) an active transport mechanism responsible for the transfer of amino acids against a concentration gradient is in operation probably during the entire pregnancy, (e) essential and nonessential amino acids are transported across the placental barrier indiscriminately, and (f) maternal diabetes and the route of delivery do not affect the concentrations of free amino acids on both the fetal and maternal side.