To evaluate the response of an epithelial barrier to a moderate but sustained oxidative stress, we cultured monolayers of Madin Darby canine kidney cells on microporous filters and exposed them to the hypoxanthine-xanthine oxidase (HX-XO) reaction. The transepithelial permeability coefficient for mannitol (Pm) was assessed as a marker of paracellular permeability. When the oxidative stress was limited in intensity and duration (production of 10 nmol/ml/min O⁻₂ with generation of 467 ± 30 nmol/ml H₂O₂ over 1 h), we observed an increase of Pm with a delay of several hours (324 ± 65 % of baseline by 6 h, P < 0.005). There was complete return to control values by 24 h. The increase of Pm did not appear to be related to a depletion of cellular ATP. Protein kinase C (PKC) activity did not increase, and the rise in Pm was not prevented by CGP 41 251, a specific inhibitor of PKC. By contrast, CGP 41 251 inhibited the Pm increase that was elicited by PDBU, a phorbol ester that activates PKC. In our model, we conclude that a reversible increase of paracellular permeability occurs after oxidative stress independently of ATP depletion or PKC activation. Other, as yet unknown mechanisms have to be involved in this process.