Plasma and erythrocyte (RBC) free amino acids and plasma peptide and serum protein amino acid concentrations and arteriovenous (A/V) differences across the hindlimbs were determined on growing Holstein steer calves (130 kg) fed purified diets containing scy protein or urea as the sole source of dietary nitrogen. Animals were fed at hourly intervals in a constantly lighted environment in order to achieve near ‘steady-state’ metabolic conditions. Experimental trials lasted 30 days with blood samples collected on day 10 and day 30 of each trial. Plasma and Plasma and RBC free amino acids were determined from filtrates obtained by deproteinization with sulfosalicylic acid. Peptide amino acids were determined from filtrates obtained by deproteinization with sulfosalicylic acid. Peptide amino acids were determined from 4N methanesulfonic acid hydrolyzed plasma filtrates. Serum proteins were grossly separated by ion-exchange chromatography. Specific protein fractions were then hydrolyzed and analyzed for amino acid concentrations. Urea resulted in the reduction of plasma free amino acid levels due to a 30% decrease in EAA and a 16% decrease in NEAA. All EAA except MET and HIS were significantly depressed. Concentrations of amino acids in blood from the RBC were generally lower of amino acids in blood from the RBC were generally lower than from plasma for all amino acids, however, HIS and ASP were always more concentrated in the RBC. Urea significantly depressed RBC concentration of THR, VAL, MET, ILE, LEU AND PHE, however, most NEAA in the RBC were unaffected by dietary treatment. Exceptions were a large increase in GLY and decrease of ALA in the RBC of urea-fed animals. Net A/V differences across the hindlimb of soy-fed animals was positive for most plasma free amino acids. GLU, GLY and CYS accounted for all of the amino acid release. Urea feeding resulted in a small negative amino acid net A/V difference due to decreased uptakes and/or increased outputs by the hindlimb of several amino acids relative to soy-fed animals. The RBC hindlimb fluxes of several amino acids were altered by dietary treatment. The BCAA changed from large outputs in soy-fed animals so essentially a zero flux in urea-fed animals. In contrast, GLY changed from a large uptake in soy treatments to a large output when urea was fed. The free amino acid data from this experiment indicate that the urea-fed calves were subject to diets deficient in total protein and amino acids rather than specific amino acid differences. Negative A/V fluxes in urea-fed calves indicate muscle protein breakdown. Time effects were generally nonsignificant although a trend towards increased amino acid output across the hindlimb in urea-fed calves was apparent at 30 days. This implies that adaptation under these conditions was not occurring but rather a more severe deficiency state was encountered over time. These data also show further interrelationships between GLY and the neutral SCAA in altered nutritional states. Plasma peptides exhibited amino acid concentrations approximately 30% greater than whole blood free amino acids. HIS was significantly depressed and a tendency for lower levels of most other amino acids was noted in urea treatments. Peptide hindlimb exchanges were variable and nonsignificant with the exceptions of GLU, LYS, HIS and VAL uptakes in soy-fed animals and ILE in urea-fed animals. Peptides may, thus, be acting as a supplemental source of several amino acids to muscle tissue. Diet had little effect on amino acid composition of blood protein fraction I (primarily globulins) and fraction II (primarily albumin). Hindlimb amino acid exchanges of both fractions in soy-fed animals were inconsistent and non-significant. In contrast, the hindlimb of urea-fed animals removed large quantities of amino acids from both fractions with fractions II making the greatest contribution. Patterns of amino acid uptake closely resemble molar ratio within each fraction indicating whole protein uptake. These data show striking evidence of increased uptake of amino acids from blood proteins in calves fed urea purified diets. This may represent adaptation by muscle tissue of the animal to protein-deficient diets.