Following an overnight fast, either 15N-labeled glycine, or ammonium chloride, or L-aspartate was orally administered to human volunteers in equal hourly aliquots for 8-12 h. In a parallel series of experiments, [15N]glycine and [15N]ammonium chloride were given intravenously at a constant rate for 10-12 h. The amount of 15N given ranged between 2.7 and 27.3 mg/h. Five hundred milligrams of sodium benzoate were given hourly. Venous blood and urine samples were collected serially. The 15N enrichment of the plasma amino nitrogen, urinary ammonia, hippuric acid, and urea were determined. Irrespective of the 15N carrier used or the route of administration, the plasma amino nitrogen, urinary ammonia, and hippuric acid 15N enrichments approached a plateau within 6-10 h. We concluded that human [15N]glycine metabolism was not atypical of amino acids and that mixing, rather than nitrogen interchange reactions, determines the rate in attaining an approximate plateau.