1. The effect of intravenous infusion of L-lysine and L-arginine on the tubular reabsorption of dibasic amino acids and cystine was studied in normal individuals and in homozygous and heterozygous subjects with cystinuria. 2. The control subjects reabsorbed almost all filtered lysine and arginine until the filtered load was elevated about fourfold. With further increased loads the tubular reabsorption began to fall and tended to approach a maximum reabsorption rate. By contrast, the homozygous subjects could not reabsorb the elevated amino acid beyond the endogenous capacity until the filtered load was increased seven- to ten-fold. When the filtered load was further increased, tubular reabsorption proceeded at the normal rate in the cystinuric patients. 3. These findings may be explained by a low-capacity transport system, which acts at low substrate concentrations, being defective in the cystinuric subjects, while a high-capacity transport system, which predominates at high substrate concentrations, remains intact. 4. Lysine and arginine infusion depressed the percentage tubular reabsorption of other dibasic amino acids and cystine both in the control and the cystinuric subjects. In the control subjects the amino acid infusion caused a gradual linear fall in the fractional reabsorption of the dibasic amino acids and cystine, whereas the depressed reabsorption of the dibasic amono acids in the cystinuric patients returned to that observed under the endogenous condition when the filtered load was high. The amino acid load caused only a gradual decrease in cystine reabsorption in the cystinuric patients. 5. In the heterozygous subjects the slope of the titration curves and the depression of the tubular reabsorption were intermediate between those of the control and homozygous subjects.