1. An autoradiographic technique is described whereby the cellular location of tritiated amino acids can be determined following uptake by rabbit ileal mucosa. 2. Stirring solutions in contact with the intestinal mucosa during measurement of rapid influx changes the quantity, but not the distribution, of alanine taken up by the tissue. 3. Conditions predicted to favour either a high affinity system (Ly1) or a low affinity system (Ly2) were used to measure lysine distribution following uptake. Maximal uptake for both transport systems occurred in fully differentiated enterocytes at the tips of villi. Initial maturation of the Ly1 system, which was slow, was followed by a rapid phase of development. The Ly2 system lacked this rapid phase of late development. 4. The cellular distribution of alanine entering on a low affinity Na-independent neutral amino acid carrier closely resembles that determine for Ly1 system for lysine entry. 5. Arginine is a potent inhibitor of lysine uptake through the Ly2 system. Little or no diffusion of lysine appears to take place into rabbit ileal enterocytes. 6. The different distribution of the high and low affinity systems for lysine transport provides further support for their independent existence. It also suggests that more than one message exists fo the switching on of amino acid transport function in differentiating enterocytes.