Gas chromatography--mass spectrometry of plasma amino acids has been used to determine the 15N enrichments of plasma glycine and alanine in rabbits in different metabolic states. Isotope-enrichment time-decay curves of plasma amino acids were linear over the course of the measurements after intravenous administration of a single dose of 15N-amino acid. Glycine and alanine pools and turnover rate constants were estimated from decay data. The effects of diurnal variation and fasting on glycine and alanine pool sizes, turnover rates, and flux in rabbits were studied to provide information on the effect of metabolic stress on amino acid kinetics in the whole body. The observations suggests that the transport of systemic glycine or alanine into the hepatocyte is under the control of a regulatory mechanism that compensates for decrease in the extracellular levels of the amino acids by enhancing the activity of the transport system. The volumes of the glycine and alanine pools were found to correspond to the extracellular space of rabbits, and the glycine and alanine pools can be identified as extracellular. We conclude that the plasma glycine and alanine 15N isotope-enrichment time-decay curves over the 1st h after a single intravenous dose of the amino acid represent mainly the hepatic uptake of glycine and alanine from the extracellular pool.