When L-phenylalanine is present in the incubation fluid, the epithelial cells of intestinal mucosa of semistarved albino rat (Wlstar strain) concentrate the amino acid and absorb water. A decrease in K concentration and an increase in Na quantity is contemporaneously occurring. The net entrance of water into the epithelial cells corresponds, osmotically, to the net gain in ions and L-phenylalanine. L-phenylalanine which enters the intestinal cells is presumably under an osmotically active form.