1. The kinetics of 2-min uptake of l-lysine and l-lysyl-l-lysine have been studied by using rings of everted hamster intestine in vitro, and values for Kt and Vmax, established.
2. On a molar basis, mediated uptake was more rapid for the amino acid than for the peptide. Non-mediated uptake was more rapid for the peptide than for the amino acid.
3. Comparison of relative rates of uptake of lysine from equivalent solutions of lysine and lysyl-lysine showed that at low concentrations, uptake of lysine was less rapid from the peptide than from the amino acid, whereas at high concentrations, uptake of lysine was more rapid from the peptide than from the amino acid. This type of effect of concentration on relative rates of uptake from equivalent solutions of amino acid and peptide has not previously been described.