Peptide, plasma free, erythrocyte free and serum protein amino acid arteriovenous differences were quantified across the gastrointestinal tract (GIT) and liver of growing calves during a "steady state" and fasting metabolism. Six Holstein steer calves (136 kg) surgically equipped with cannulae in the aorta, portal vein and hepatic vein were maintained under constant light and fed a natural diet as 24 hourly meals per day. Blood was obtained simultaneously from all cannulae at 9 to 12 d postsurgery during the midpoint of a 1-hr feeding interval and following a 72-hr fast. Most amino acids were added to plasma by the GIT but fasting reduced the magnitude of this addition. GLN and GLU were the only plasma amino acids removed by the GIT. The liver removed a large portion of amino acids in portal plasma but a net splanchnic plasma free amino acid output occurred. Hepatic removal of amino acids, particularly VAL, ILE and LEU, increased after fasting. The GIT generally removed amino acids from the erythrocyte while the liver contributed amino acids to this pool. Peptide amino acids increased as blood crossed the GIT. Responses paralleled those of plasma free amino acids in direction but were threefold greater for peptide amino acids. A net output of peptide amino acids from the splanchnic bed was observed. Generally amino acids were added to the primarily globulin, albumin and mixed protein serum fractions while the pure albumin fraction contributed amino acids to the GIT. Fasting reversed the direction of response in the four fractions monitored. Arteriovenous differences for amino acids in two largely albumin fractions responded oppositely indicating a potential transport role of other proteins in these fractions. A more variable response was observed across the liver with direction and magnitude dependent upon individual amino acids. Fasting resulted in a release of amino acids by the liver into all four fractions. Several blood pools appear to be involved in the interorgan transport of amino acids. The extent and direction of transport varies with tissue, blood pool, nutritional status and amino acid. Quantitatively, peptides appear to be highly involved in interorgan movement of amino acids. / Ph. D.