The compartments of amino-acid entry into intestinal fragments incubated in vitro were determined. Four compartments seem to exist: permeation of the extracellular space; free diffusion into the muscular part of the tissue; facilitated fiffusion into the mucosal cells, dependent on the presence of Na in the incubation medium, but independent of cellular energy supplies; active transport into the mucosa, dependent on the presence of Na and the availability of cellular energy. Competition between amino-acids of similar molecular structure is shown to exist for the facilitated diffusion step and for the active transport mechanism. Both the active transport and the facilitated diffusion processes are lost when the tissue loses its viability.