Somatotropin (ST) administration enhances protein deposition in well-nourished, growing animals. To determine whether the anabolic effect is due to an increase in protein synthesis or a decrease in proteolysis, pair-fed, weight-matched (∼20 kg) growing swine were treated with porcine ST (150 μg ⋅ kg⁻¹⋅ day⁻¹, n = 6) or diluent ( n = 6) for 7 days. Whole body leucine appearance (R_a), nonoxidative leucine disposal (NOLD), urea production, and leucine oxidation, as well as tissue protein synthesis (K_s), were determined in the fed steady state using primed continuous infusions of [¹³C]leucine, [¹³C]bicarbonate, and [¹⁵N₂]urea. ST treatment increased the efficiency with which the diet was used for growth. ST treatment also increased plasma insulin-like growth factor I (+100%) and insulin (+125%) concentrations and decreased plasma urea nitrogen concentrations (−53%). ST-treated pigs had lower leucine R_a(−33%), leucine oxidation (−63%), and urea production (−70%). However, ST treatment altered neither NOLD nor K_sin the longissimus dorsi, semitendinosus, or gastrocnemius muscles, liver, or jejunum. The results suggest that in the fed state, ST treatment of growing swine increases protein deposition primarily through a suppression of protein degradation and amino acid catabolism rather than a stimulation of protein synthesis.