Valine and leucine kinetics were studied in four young healthy men in the postabsorptive state with a 4-h primed infusion of either L-[1-13C,15N] valine or L-[1-13C,15N]-leucine. For 1 wk before each infusion study each subject consumed a diet that provided an adequate amount of energy and 1.6 kg/day of protein. During infusion of tracer, plasma valine or leucine, and expired 13CO2 reached isotopic steady state by 2 h. The valine and leucine carbon fluxes (mean +/- SE) were 80.3 +/- 1.2 and 86.6 +/- 2.0 mumol kg-1h-1, respectively, consistent with the lesser content of valine compared with leucine in body protein. Valine and leucine oxidation rates were 11.8 +/- 0.6 and 15.9 +/- 1.1 mumol kg-1h-1, respectively. From these values and values for valine and leucine nitrogen flux, the rates of valine and leucine transamination were calculated. Valine and leucine deamination were 84.0 +/- 3.5 and 103.0 +/- 6.5 mumol kg-1h-1, and values for reamination were 72.2 +/- 3.3 and 87.1 +/- 7.5 mumol kg-1h-1, respectively. Thus, the patterns of valine and leucine catabolism are similar. However, when the plasma substrate levels are used to estimate transamination rate constants, we estimate that the transamination equilibrium favors leucine transamination over valine by 5-fold.