1. There are two saturable transport processes in the monkey small intestine for glycyl-l-leucine, one with Kmax 1 μmol min−1 g−1 wet weight of tissue and an affinity constant (kt) of 5 mmol/l, and the other with Vmax. 3·9 μmol min−1 g−1 wet weight of tissue and kt 33 mmol/l.

2. Glycyl-l-leucine uptake is inhibited by a wide variety of amino acids, although to a variable extent. The inhibition was shown to be competitive with leucine used as the representative amino acid. Phenylalanine, methionine, alanine and leucine are the most potent in their inhibitory action.

3. The effect of various amino acids on the hydrolysis of glycyl-l-leucine by particulate and cytosol fractions of monkey small intestine was studied. All the amino acids, except glycine, proline, alanine and glutamic acid, inhibit both the particulate and cytosol glycyl-l-leucine hydrolase activities. In general, the cytosol enzyme is more susceptible to amino acid inhibition than the particulate enzyme.

4. There is no correlation between the effects of amino acids on glycyl-l-leucine uptake and hydrolysis of glycyl-l-leucine by either particulate or cytosol fraction.

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