Hydrogen peroxide (8202) is produced in the intestinal tract by inflammatory cells or by reperfusion of intestinal tissues following ischémie injury. We have examined the prolonged effects of H2O2 on human colonie T84 cells grown to confluence on permeable supports. Monolayers were mounted in modified Ussing chambers and the short-circuit current (Isc), a reflection of eletrogenic cr secretion, was measured. The basal Isc was 3±1 μA/cm2 and increased to 36±2 μA/cm2 following forskolin (10 μM) treatment. Subsequent addition of H2O2 (0.5 mM to both sides) at the peak of the forskolin response further increased the Isc transiently, but this was followed by a sustained inhibition (∼40%) at >30 min. Pretreatment with H2O2 for 30 min inhibited the forskolin-induced Isc response (37% at 0.5 mM with complete inhibition at 5 mM). To determine the mechanism underlying this inhibitory response, we examined the effects of H2O2 (0.5 mM) on Na/K-ATPase activity, measured as Isc after selectively permeabilizing the apical membrane to monovalent ions with amphotencin B (10 μM). Under these conditions, the Isc was inhibited 76% in monolayers treated with H2O2 compared to control. We also evaluated the effect of H2O2 on the apical membrane Cf conductance by measuring the Isc across monolayers basolaterally permeabilized with nystatin (300 ug/ml). In the presence of forskolin and a Cl- concentration gradient (120:6 mM, mucosal:serosal), 30 min treatment with H2O2 (05 mM) inhibited 65% of the Isc. We conclude that prolonged exposure to H2O2 inhibits Na/K-ATPase activity and Cl- channel activity thus reducing Cl- secretion across colonie epithelium (NIDDK-01935).