The concentrations of amino acids in amniotic fluid have been used in the prenatal diagnosis of several inherited metabolic disorders. However, previous studies have usually examined only a small number of control amniotic fluid samples. We have, therefore, measured the amino acids in amniotic fluid samples from 183 normal pregnancies between the 13th and the 23rd wk gestation of women ranging in age from 17 to 43 yr. The concentrations of Ala, Lys, Val, Glu, Pro, Thr, and Gly, in descending order, accounted for about 70% of the amino acids in amniotic fluids. A negative correlation with gestational age (-0.34 to -0.24) was found for Leu, Val, Ile, Phe, Lys, Ala, Asp, Tyr, Glu, and Pro, with Leu showing the greatest rate of change. The concentration of Gln increased slightly (r = 0.18), whereas the other amino acids did not change significantly during this period. Statistically significant positive correlations, at all gestational ages, were observed among Val, Leu, and Ile. These branched-chain amino acids also correlated positively with Phe, Lys, Asp, Thr, Ser, Glu, Pro, Gly, Ala, and Tyr, and the amino acids within this group correlated with each other. Additionally, strong positive correlations were observed between Phe and Tyr and between Gly and Ser. No significant correlations were found between any of the amino acids and maternal age or fetal sex. Significant positive correlations between amino acids may be indicative of common transport or degradative pathways and suggest that maintenance of specific relative concentrations in amniotic fluid may be essential for normal fetal development.(ABSTRACT TRUNCATED AT 250 WORDS)