The efflux of the nutritionally essential amino acid, L-lysine from the basolateral (BL) membrane was characterized in human cultured intestinal cells (Caco-2) grown and differentiated on permeable filter supports. Cells were loaded by incubating with 3H-lysine from the apical (AP) side in the absence of sodium (substituted with choline) in the BL medium; under these conditions, cells accumulated lysine in the intracellular soluble pool to 10- to 20-fold the extracellular concentration. L-Lysine efflux in the BL medium was then followed, and initial rates of efflux were calculated under different experimental conditions. L-Lysine efflux exhibited a strong energy dependence. The presence of an inwardly directed gradient of sodium or lithium stimulated lysine efflux; ouabain reduced efflux in both sodium- and lithium-containing medium. When zwitterionic or cationic amino acids were added to the BL medium, L-lysine efflux was strongly stimulated. The most efficient trans-stimulating amino acids were L-leucine > L-methionine = L-ornithine = L-arginine. In the presence of trans-stimulating amino acids in the BL medium, L-lysine efflux exhibited energy independence and was not affected by the presence of a sodium gradient. In addition, the sensitivity, of efflux to N-ethylmaleimide was different in the absence or in the presence of amino acids in the BL medium. These results suggest that different mechanisms may operate in the BL efflux of L-lysine from human intestinal epithelial cells, depending on the extracellular availability of other amino acids, to guarantee optimal bioavailability of this essential amino acid both in the postprandial absorptive period and between meals.