1. The renal handling of lysine was studied during the intravenous infusion of a lysine load given in such a way that the plasma lysine concentration was constantly increasing. Theoretical renal thresholds, and maximum rates of renal tubular reabsorption of lysine, have been determined in three normal adults, five homozygous cystinuric patients, and three persons, two ‘completely’ and the third ‘incompletely’ recessive, heterozygotes for cystinuria. All the cystinuric patients were found to have thresholds at less than 1 mg of lysine/100 ml of plasma; the thresholds of all three heterozygotes fell within the same range as those of the three normal persons, that is, at values only two to three times the fasting plasma lysine concentration. 2. Not only was the lysine load shown to greatly increase the excretion of cystine, arginine and ornithine in the normal and heterozygous persons, but it was also shown to increase the already high clearance of these three amino acids in three of the homozygous cystinuric patients. 3. It was shown that a water diuresis alone will not cause increased excretion of any amino acids except taurine.