Insulin stimulates muscle protein synthesis when the levels of total amino acids, or at least the essential amino acids, are at or above their postabsorptive concentrations. Among the essential amino acids, branched-chain amino acids (BCAA) have the primarily role in stimulating muscle protein synthesis, and are commonly sought alone to stimulate muscle protein synthesis in humans. Fourteen healthy, young subjects were studied before and after insulin infusion to examine whether insulin stimulates muscle protein synthesis in relation to the availability of BCAA alone. Half of the subjects were studied in the presence of postabsorptive BCAA concentrations (Control), while the other half in the presence of increased plasma BCAA (BCAA). When compared to that prior to the initiation of the insulin infusion, fractional synthesis rate of muscle protein (%/hour) did not change (P > 0 .05) during insulin in either the Control (0.04 ± 0.01 vs 0.05 ± 0.01) or the BCAA (0.05 ± 0.02 vs 0.05 ± 0.01) experiments. Insulin decreased (P < 0.01) whole-body phenylalanine rate of appearance (umol/kg/min), indicating suppression of muscle proteolysis, both in the Control (1.02 ± 0.04 vs 0.76 ± 0.04) and the BCAA (0.89 ± 0.07 vs 0.61 ± 0.03) experiments, but the change was not different between the two experiments (P > 0.05). In conclusion, insulin does not stimulate muscle protein synthesis in the presence of increased circulating levels of plasma BCAA alone. Insulin's suppressive effect on proteolysis is observed independently of the levels of circulating plasma BCAA.