1. Anterior tibial muscle protein synthesis in seven healthy postabsorptive men was determined from increases in muscle protein bound leucine enrichment during a primed continuous infusion of L-[1-13C]leucine. Biopsies were taken 30 min after the beginning of leucine infusion (when plasma 13C enrichment was steady), 240 min later during continued fasting and again after 240 min of infusion of a mixed amino acid solution which increased plasma total amino acid concentrations by 37%. The mean enrichment of 13C in plasma alpha-ketoisocaproate was used as an index of the enrichment of the precursor pool for leucine metabolism. 2. Anterior tibial muscle mixed protein synthetic rate during fasting was 0.055 (SD 0.008)%/h and this increased by an average of 35% during infusion of mixed amino acid to 0.074 (SD 0.021)%/h (P less than 0.05). 3. Whole-body protein breakdown (expressed as the rate of endogenous leucine appearance in plasma) was 121 (SD 8) mumol h-1 kg-1 during fasting and decreased (P less than 0.01) by an average of 12% during amino acid infusion. Leucine oxidation was 18 (SD 3) mumol h-1 kg-1 during fasting and increased (P less than 0.001) by 89% during amino acid infusion. Whole-body protein synthesis (non-oxidative leucine disappearance) was 104 (SD 6) mumol h-1 kg-1 during fasting and rose by 13% (P less than 0.001) during mixed amino acid infusion. 4. 13C enrichment of muscle free leucine was only 61 (SD 19)% of that in plasma alpha-ketoisocaproate and this increased to 74 (SD 16)% (P less than 0.02) during mixed amino acid infusion. 5. The results suggest that increased availability of amino acids reverses whole-body protein balance from negative to positive and a major component of this is the increase in muscle protein synthesis.