The objectives are: To describe quantitatively the rates of movements between organs and of net organ metabolism of amino acids and related compounds with respect: 1) to the mode of transport of amino acids, whether as free amino acids, proteins, or peptides, carried in plasma or blood cells; 2) to the effects of food ingestion in terms of the diurnal patterns related to the time of ingestion, route of ingestion, and composition of the diet; 3) to the effects of protein depletion and repletion; and 4) in depleted subjects to the interrelations between interorgan amino acid movements, plasma amino acid patterns, changes in extra and intracellular fluid volumes, nitrogen and potassium retention and excretion of urea and creatinine. Amino acid movements will be studied in unanesthetized dogs with catheters in appropriate blood vessels. Arterial blood, and blood from the venous drainage of the gut, liver, hind limb and other regions will be sampled over extended periods, under control conditions and during and after administration of test diets or intravenous alimentation. Concentrations in plasma and rbc's will be determined, blood flow will be measured, and net organ or regional metabolism will be calculated for each compound, and related to rates of urea production and of net synthesis or breakdown of protein. In depleted, chronically ill human subjects, arterial-venous amino acid concentration differences across the arm or leg will be measured prior to and during repletion therapy with oral or intravenous feeding. They will be related to changes in body composition and nitrogen and electrolyte balance in order to differentiate nutritional from other etiologic factors and to provide improved criteria for evaluation of therapy.