W e have previously shown that Na(+)-coupled transport of glucose and amino acids across the apical membrane of intestinal absorptive cells is accompanied by alterations in cytoskeletal structure and altered sieving of small hydrophilic solutes by tight junctions. Here we report that in response to the essential amino acid L-tryptophan at lumenal concentrations likely to be supraphysiological (1 mM or greater), these responses are so exaggerated as to induce disruption of tight junctions and transepithelial macromolecular leaks. Since these effects of L-tryptophan are energy and Na+ dependent and occur with mucosal but not serosal exposure to L-tryptophan, it appears they are triggered by activation of a Na(+)-nutrient cotransporter in the apical membrane of absorptive cells rather than by the presence of an unidentified trace contaminant. Our findings suggest the possibility that dietary supplementation by L-tryptophan may result in loss of the intestinal epithelial barrier to dietary antigens. We speculate that such a response to supraphysiologic tryptophan may contribute, in part, to the recently recognized curious tryptophan-induced eosinophilia myalgia syndrome.