The attachment of a benzyloxy group to the 5-position of the indole system of the LAT1 (SLC7A5) substrate aminoacid l-Trp produced a moderately potent inhibitor of LAT1-mediated leucine transport in HT-29 cells, while no inhibition of LAT1 was observed for 4-, 6-, or 7-benzyloxy-l-Trp at concentrations below 100 μM. None of these benzyloxy-l-Trp residues was found to be a transport substrate. The