Plasma amino acid concentrations decrease in pregnancy despite increased requirements for both maternal and fetal protein accretion. There are few published data on changes in amino acid turnover in pregnancy. The aim of this study is to test the hypotheses that (1) whole body nonoxidative leucine disposal (NOLD) is higher and (2) whole body oxidative leucine disposal (OLD) is lower in the third trimester of pregnancy than in the nonpregnant state. After an overnight fast 8 pregnant women between 33 and 35 weeks gestation had a primed infusion of labeled [1-(13)C] leucine and a prime dose of NaH(13)CO(3). Carbon dioxide production was measured using indirect calorimetry. Gas chromatography-mass spectrometry was used to determine (13)CO(2) enrichment of expired air and oxidative and nonoxidative leucine turnover by measuring (13)C keto-isocaproate plasma enrichment, which reflects intracellular leucine enrichment. Women acted as their own controls after the puerperium. Whole body leucine turnover expressed per unit body weight was increased in pregnancy (median [interquartile range or IQR]: pregnant = 103.1 [14.9] v nonpregnant = 90.1 [10.9] micromol/kg/h). The mean (+/-SD) of the differences was 11.4 +/- 5.6 micromol/kg/h, P =.0006. NOLD was increased in pregnancy (pregnant = 86.8 [10.1] v nonpregnant = 73.3 [9.5] micromol/kg/h). The mean (+/-SD) of the differences was 10.6 +/- 5.4 micromol/kg/h, P =.0008. OLD was not significantly altered in pregnancy (pregnant = 17.3 [4.5] v nonpregnant = 15.91 [2.4] micromol/kg/h). The mean (+/-SD) of the differences was 0.84 +/- 1.94 micromol/kg/h, P =.26. In conclusion, women have significantly higher NOLD in the third trimester of pregnancy than when not pregnant but there are no significant changes in OLD.