1. The uptake of l-methionine and glycine as free amino acids, and from their dipeptides by everted rings of rat small intestine in vitro has been investigated. The concentrations used covered a wide range, including values likely to be near those found in the lumen of the intestine.
2. Though no intact peptides were found in the mucosal cells, evidence was obtained which showed that hydrolysis of the peptides was cellular at all concentrations. Total hydrolysis of peptides by the intestine was very great in relation to amino acid uptake over very short incubation times, suggesting that much hydrolysis took place superficially.
3. Except at the lowest concentrations, the rates of uptake of amino acids from the peptides were more rapid than from the equivalent amino acid mixtures. Competition for uptake between glycine and methionine was avoided when they were presented in the form of l-methionylglycine.
4. Anoxia inhibited uptake of methionine from free l-methionine and from l-methionyl-l-methionine. It also inhibited hydrolysis of l-methionyl-l-methionine by intact intestine, but not by intestinal homogenates, suggesting that peptide uptake may be energy-dependent. The l-amino acid oxidase of snake venom, which destroys l-methionine but has no effect on glycine or on the peptides studied, inhibited methionine uptake from peptides when present at high concentrations, suggesting that a major site of hydrolysis is enzyme-accessible.
5. It is suggested that there may be two modes of uptake of amino acids from oligopeptides: (1) surface hydrolysis by mechanisms closely linked to the amino acid entry mechanisms, and (2) peptide entry into the mucosal cells by a special mechanism, followed by intracellular hydrolysis.