This report makes use of a recent developed method with stable isotope and gas chromatography-mass spectrometry to determine the disappearance of labeled amino acids from plasma samples after iv administration of a single dose (93.3-mumoles/kg body weight) of [15N]glycine (92% N) in neonates. [15N]Glycine measurements were studied three times in each of six preterm infants at different gestational age and twice in two full-term infants. The first study was carried out in all infants 5 to 32 hr after delivery, the second study was performed on the third day of life, and the third study, included only the preterm infants, was at the age of 25 to 29 days. The isotope disappearance curves were linear within the first hr after [15N]glycine administration and represent mainly the hepatic uptake of glycine from the extracellular pool. The volume of glycine pool varied from the day of birth to 4 wk of life. Turnover rate constants of glycine ranging from 1.35 to 2.19 hr-1 were observed in preterm and term infants during the first 32 hr. Significant increases in turnover rate constants were noted on the third day of life in most infants. At 3 to 4 wk of life, statistically significant differences in pool size and turnover rate constants were obtained. An increase of 2- to 3-fold in turnover rate constants was observed as compared to day of birth. Pool sizes declined by 50%, but the resulted fluxes remained almost unchanged during the neonatal period. At the age of 3 to 4 wk, all these infants showed similar kinetic data as in adults.