A study was carried out with 12 young men to examine the relationships between the intake of leucine and indices of leucine kinetics, using L-[1-13C]leucine as a tracer. Six subjects received L-amino acid diets during 7-day periods supplying leucine in the range of 79 to 20 mg.kg-1.day-1 (Group I) and another six subjects (Group II) received leucine intakes ranging from 20 to 4 mg.kg-1.day-1. Estimations were made of leucine kinetics, at the end of each diet period, when subjects were receiving small isonitrogenous, isocaloric meals during the isotope infusion period. Leucine flux declined with reduced leucine intake and leucine oxidation tended not to change at intakes below 20 mg.kg-1.day-1 (slope not statistically different than zero). Plasma valine increased markedly with further restriction in leucine intake below this level. The daily mass balance of leucine, estimated from the difference between intake and oxidation, became negative at an intake of about 20 mg.kg-1.day-1. These findings are discussed in relation to the published mean and upper range of requirement for leucine in healthy adults, currently taken to be 11 mg.kg-1.day-1 and 14 mg.kg-1.day-1, respectively.