Healthy young men participated in a study designed to explore the effects of decreasing dietary lysine content on plasma amino acid concentrations and lysine kinetics, studied with L-[1-13C]lysine as tracer. Diets provided adequate energy and the equivalent (N X 6.25) of 0.8 g protein kg-1.day-1 as a synthetic L-amino acid mixture simulating egg protein. Lysine intake was reduced every 7 days. Changes in plasma amino acids suggested that effects characteristic of a dietary lysine inadequacy were prevented by consuming more than 32 mg lysine.kg-1 day-1. Primed, continuous intravenous infusions of L-[1-13C]lysine, at the end of each diet period while subjects were in the fed state, showed that as dietary lysine was reduced, 13C enrichment increased in plasma lysine and decreased in expired CO2. It was estimated that lysine oxidation exceeded, by 4.4 mg kg-1 day-1, the lysine intake of 20 mg kg-1.day-1 indicating that the lysine required for body protein maintenance would probably exceed this latter value. These results are discussed in relation to the physiological requirement in adults for lysine, currently accepted to be met by an intake of 12 mg kg-1.day-1. which is assumed to be the upper range of the lysine requirement for this population group.