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. 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 < 1/2 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. Total uptake of L-leucyl-L-leucine at pH 5 is probably the result of at least 2 processes: uptake of intact peptide by 1 or more mechanisms, and also hydrolysis followed by uptake of free amino acid. At most concentrations, > 1/2 the mediated uptake of L-leucyl-L-leucine was in the form of intact peptide. 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-leucyl-L-leucine. The existence of > 1 dipeptide uptake system in the small intestine seems probable.