Technical Abstract: Growth hormone (GH) treatment enhances metabolic efficiency by minimizing protein loss during fasting and maximizing protein gain during meal absorption. To determine the tissue-specific response to GH treatment, amino acid metabolism was measured in hindquarter (HQ) and portal-drained vicera (PDV) following 7d GH treatment (150 micro g/kg/d). Fully fed pigs (n=20) were infused for 4h with [1-13C] phenylalanine (Phe) intraduodenally and 2H5 Phe intravenously. Pigs were weight-matched and pair-fed a high protein diet. Simultaneous blood flow measurements, from portal vein and caudal aorta flow probes, and blood samples, from carotid artery, portal vein, and vena cava catheters, were obtained at baseline and during steady state conditions. Results indicate GH caused a 2-fold increase in HQ blood flow and an increase in HQ Phe uptake with no effects in PDV. GH increased the first-pass Phe utilization by the gut. Regardless of treatment, a greater percentage of dietary vs. arterial Phe was utilized by the gut. HQ and PDV proteolysis rates were unaffected by GH although whole body proteolysis was reduced. GH caused an increase in HQ, but not PDV, protein synthesis, even though whole body synthesis was unaffected by GH. Phe oxidation was unaffected by GH in HQ and PDV, even though whole body oxidation was reduced. All of these changes contribute to an improvement in protein balance with GH treatment in growing pigs.