1. The intestinal transport of oligopeptides containing either lysine or arginine has been compared with that of the corresponding free amino acids in six homozygous cystinuric patients and in six normal adult subjects by use of the oral tolerance test technique.
2. No difference was found in the absorption of lysine from l-lysylglycine and an equivalent mixture of free lysine and glycine.
3. In comparisons of serum increments of lysine and arginine after oral casein and an equivalent free amino acid mixture there was no difference in the case of lysine but rise of serum arginine was higher in three cystinuric patients after the whole protein than after the amino acids.
4. Studies of urinary piperidine and pyrrolidine output in a single cystinuric patient supported the results of the tolerance tests.
5. Absorption rates of the dipeptides l-lysylglycine and l-arginyl-l-aspartate from an isolated loop of rat gut are compared with those of an equivalent free amino acid mixture. No difference was found for the former peptide, but in the latter absorption rates of arginine were higher after the amino acid mixture.