The concentration of free amino acids in amniotic fluids, obtained from 27 normal mothers between the eighth and 22nd weeks of gestation, was determined by cation-exchange chromatography. Of the 29 free amino acids detected, three—γ-aminobutyric acid, ethanolamine, and homocarnosine— had not previously been found in amniotic fluid. The seven most common (in descending order) are alanine, glutamic acid-glutamine, lysine, proline, threonine, glycine, and valine. These seven comprise 70% of the total free amino acids present. All of the amino acids found in amniotic fluid have been found in serum and urine from fetuses and newborns and in the mother's urine. However, three of the amino acids found in amniotic fluid—ethanolamine, hydroxyproline, and homocarnosine—have not been reported to be present in serum from mothers.