Experiments in humans and rodents using oral doses of glycine and phenylalanine have suggested that the metabolism of these amino acids contributes to urinary oxalate excretion. To better define this contribution, we have examined the primed, constant infusion of [1-(13)C(1)] phenylalanine and [1,2-(13)C(2)] glycine in the postabsorptive state in healthy adults. Subjects were infused for 5 hours, hourly urines were collected, and blood was drawn every 30 minutes. Ion chromatography/mass spectrometry was used to measure [(13)C] enrichment in urinary oxalate, glycolate, and hippurate; and the enrichment of (13)C-amino acids in plasma samples was measured by gas chromatography/mass spectrometry. Following infusion with either 6 μmol/(kg h) [1-(13)C(1)] phenylalanine or 6 μmol/(kg h) [1,2-(13)C(2)] glycine, no isotopic glycolate or oxalate was detected in urine. Based on the limits of detection of our ion chromatography/mass spectroscopy method, these data indicate that less than 0.7% of the urinary oxalate could be derived from phenylalanine catabolism and less than 5% from glycine catabolism. Infusions with high levels of [1,2-(13)C(2)] glycine, 60 μmol/(kg h), increased mean plasma glycine by 29% and the whole-body flux of glycine by 72%. Under these conditions, glycine contributed 16.0% ± 1.6% and 16.6% ± 3.2% to urinary oxalate and glycolate excretion, respectively. Experiments using cultured hepatoma cells demonstrated that only at supraphysiological levels (>1 mmol/L) did glycine and phenylalanine metabolism increase oxalate synthesis. These data suggest that glycine and phenylalanine metabolism make only minor contributions to oxalate synthesis and urinary oxalate excretion.