The effect of enteropathogenic Escherichia coli (EPEC) infection on electrophysiology of T84 cell monolayers was examined. After 18 h of infection with EPEC (E2348), transepithelial electrical resistance was decreased (30 +/- 5% of uninfected values) compared with monolayers infected with a nonpathogenic E. coli strain (104 +/- 13%). Resistance of monolayers infected with EPEC mutant strain CVD206, deficient in attaching and effacing lesion formation, was partially reduced (66 +/- 10%). In addition, permeability of EPEC-infected T84 monolayers increased compared with uninfected cells. Associated with these changes was an altered distribution of the tight junction protein, ZO-1. Taken together, these findings suggest that the barrier defect induced by EPEC was at the level of the tight junction. Adenosine 3'5'-cyclic monophosphate-stimulated chloride secretion was also diminished in EPEC-infected cells, whereas Ca2+ -dependent chloride secretion was not different from uninfected cells. These findings indicate that EPEC infection alters intestinal epithelial barrier and transport functions. Furthermore, these results provide a possible mechanism for EPEC-induced diarrheal disease.