To investigate in humans whether interconversion of leucine and its alpha-keto acid, alpha-ketoisocaproate (KIC), is rate limiting for leucine metabolism, normal volunteers were infused with independent 3H and 14C labels of leucine and KIC. Total leucine carbon (leucine + KIC) entry into the plasma space determined by using both plasma specific activities (SA) of the infused isotopes or by using either plasma SA of the leucine moieties reciprocal to those being infused (e.g., [3H]KIC and [14C]leucine during [3H]leucine and [14C]KIC infusion) were nearly identical. When [3H]leucine and [14C]KIC were infused, the ratio of 3H/14C radioactivity of leucine in plasma proteins was similar both to the ratio of plasma [3H]KIC SA/[14C]leucine SA and to the ratio of the rate of [3H]leucine/[14C]KIC infused. Estimates of leucine oxidation using expired 14CO2 and the plasma SA of the 14C moiety reciprocal to infused [14C]leucine or [14C]KIC were similar. We conclude that the plasma SA of the leucine moiety reciprocal to infused labeled leucine or KIC may reflect more accurately whole-body leucine metabolism in postabsorptive humans.