Quantitative autoradiography was used to measure the distribution of transport of leucine, lysine, and glycyl-L-leucine along the villus of the rat jejunum. A comparison was made between normal juvenile animals and ones maintained on a 5% protein isocalorific diet. Uptake of leucine, lysine, and glycyl-L-leucine was confined to the top 150 microns of the villus in control animals and initial expression of transport corresponded to a cell age of 32 h. The low-protein diet altered the distribution of transport in the same way for all three substrates, but although the position up the villus of the initial expression of transport was altered, transport always appeared in cells 32 h old. These data indicate that the age of intestinal epithelial cells is an important determinant of their ability to transport amino nitrogen. Transport kinetic data indicated that peptide uptake was initially elevated in response to the low-protein diet while free amino transport was depressed. It is concluded that a second locus of control determines the amount of transport expressed per individual cell.