The trophoblast functions of nutrient transport and protein synthesis generate high concentrations of amino acids in the placenta and in fetal blood during the second half of pregnancy, but little is known about these metabolic processes in embryonic and early fetal periods. The aim of this study is to compare the distribution of amino acids inside the first trimester gestational sac. Free amino acid concentrations were measured in homogenates of placental villi, in samples of coelomic and amniotic fluid, and in the maternal serum from 17 normal pregnancies between 7 and 11 weeks of gestation. Significant positive relationships between maternal serum and placental tissue were found for 10 amino acids, indicating that active amino acid transport and accumulation by the human syncytiotrophoblast occurs as early as 7 weeks of gestation. The transplacental flux of most amino acid transport from maternal blood to the exocoelomic cavity was against a concentration gradient. The highest placental amino acid concentrations were found for taurine, glutamic acid, glycine and alanine. The amniotic fluid contained lower mean concentration of all amino acids than coelomic fluid and maternal serum. The concentration distribution of individual amino acids in coelomic and amniotic fluid were related indicating a passive transfer through the amniotic membrane. A coelomic-maternal gradient was observed in 19 out of 24 amino acids measured and positive correlations were found between maternal serum and coelomic fluid for concentrations of alpha-aminobutyric acid, tyrosine and histidine, suggesting that these amino acids are only partially retained and/or transferred more rapidly by the early placenta.