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The parenteral administration of partial and complete protein hydrolysates is accompanied by an appreciable excretion of the infused amino acids and polypeptides in the urine (1-5). The renal loss of amino nitrogen following parenteral administration may be as great as 28 times the loss from the administration of an equivalent amount of nitrogen as whole protein orally (6). If, in addition to such urinary losses, one or more of the essential amino acids were present in "limiting" amounts or were preferentially excreted, the amount of a protein just capable of maintaining nitrogen equilibrium when given orally might be insufficient when given intravenously as a hydrolysate (6).The experiments reported here were undertaken to determine the extent of the renal loss of amino acids after their intravenous infusion. Determinations were made of the renal excretion of the eight amino acids essential for man (7) and of arginine and histidine, the urinary excretion of alpha amino nitrogen, and the blood alpha amino nitrogen following the intravenous administration to normal subjects of a 10 per cent solution of amino acids. The results indicate that the administration of protein or amino acids orally or intravenously, slowly or rapidly, and in large or small quantity is followed by the excretion in the urine of but a small portion of that administered. MATERIALS AND METHODSThe amino acid mixture 3 was made by the complete acid hydrolysis of casein, contains no peptides, is essen1The expenses of this investigation were defrayed in part by a grant from Merck and Company, Inc., Rahway, New Jersey, to Harvard University.