The amino acids of blood and urine have been investigated using chromatography on cation exchange columns in the study of a patient with idiopathic hyperglycinemia. Marked increases in concentrations of glycine, serine, alanine, isoleucine and valine were found in the plasma. These changes were not reflected in increased excretion of these amino acids in the urine (with the exception of glycine). Restriction of the dietary intake of protein resulted in a decrease in the concentrations of glycine and other amino acids in the blood and urine, and there was a concomitant decrease in the frequency and severity of episodes of acute illness. The oral administration of leucine was found to induce a decrease in the levels of a number of amino acids in the patient and in controls. Continued decrease during the 3 hours of observation was noted for serine, isoleucine and valine. A mild but progressive decrease in threonine concentration was observed in the controls, while in the patient the concentration increased after the administration of leucine. Decreased levels at 1½ hours, returning toward the fasting levels at 3 hours, were observed for alanine, taurine and glycine. These apparently normal responses to leucine loads were not mediated through increase in the urinary excretion of the amino acids involved, and the data are interpreted to indicate entry of these amino acids into cells.