Endogenous renal clearance rates of free amino acids were determined by use of automatic amino acid analyzer on ten control children, two patients with prolinuria and a Hartnup patient. Prolinuric patients showed markedly high rates in the proline and glycine clearance and almost normal clearance rates in the other amino acids. In a Hartnup patient, the clearance rates of monoamino-monocarboxylic amino acids were found to be highly elevated.