1. Preliminary observations of the effects on intestinal transport of the lipophilic properties of the amino acid side chains of a series of neutral dipeptides showed that, contrary to expectation, l-valyl-l-valine and not l-leucyl-l-leucine was the most powerful inhibitor of uptake of the hydrolysis-resistant dipeptide glycylsarcosine by hamster jejunum in vitro.

2. Investigation of the kinetic characteristics of uptake of l-leucyl-l-leucine showed that Kt and Vmax. were lower than the corresponding values for l-valyl-l-valine, suggesting a higher apparent affinity for transport and a lower maximal velocity of transport. Ki for the inhibitory effect of l-leucyl-l-leucine on uptake of glycylsarcosine was less than one-half of the Kt for l-leucyl-l-leucine, so that inhibition was stronger than that expected from the apparent affinity for transport obtained from the kinetics of uptake of the inhibitor. In spite of this, l-leucyl-l-leucine was a much less powerful inhibitor of uptake of glycylsarcosine than was l-valyl-l-valine.

3. The results suggest that total uptake of l-leucyl-l-leucine at pH 5 is the result of at least two processes: uptake of intact peptide by one or more mechanisms, and also hydrolysis followed by uptake of free amino acid. At most concentrations, more than half the mediated uptake of l-leucyl-l-leucine was in the form of intact peptide.

4. The results of experiments on competition for uptake between dipeptides were unexpected. l-leucyl-l-leucine could inhibit mediated uptake of intact glycylsarcosine completely, but glycylsarcosine could not cause complete inhibition of mediated uptake of intact l-leucyl-l-leucine. Glycylsarcosine could, however, cause complete inhibition of mediated uptake of intact l-valyl-l-valine, which in turn could cause complete inhibition of mediated uptake of intact l-leucyl-l-leucine. The existence of more than one dipeptide uptake system in the small intestine seems probable.

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