We investigated whether lysine transport would be subject to adaptive regulation in Caco-2 human intestinal cells. The activity of Lys transport in Caco-2 cells decreased with increasing incubation time with 10 mM Lys. Among the two systems involved in Lys transport, the system b0,+ component was greatly decreased by incubating cells with 10 mM Lys, whereas the system y+ component did not change. These results suggest that system b0,+ mainly contributes to the adaptive regulation of Lys transport in Caco-2 cells.