The Caco-2 cell line is a reliable in vitro model for predicting drug intestinal absorption and P-glycoprotein (P-gp)-mediated excretion in humans. Recent in vivo studies suggested the induction of P-gp as a cellular protection tool against paraquat poisoning, through the increase in its pulmonary and intestinal excretion. Thus, the aim of the present work was to evaluate P-gp expression and activity in Caco-2 cells exposed to doxorubicin (a known P-gp inducer) and to correlate these changes with paraquat toxic effects. Cytotoxicity of doxorubicin (0-100 μM) and paraquat (0-1,000 μM) was evaluated for a maximum period of 96 h. In doxorubicin-exposed cells, P-gp expression and transport activity were evaluated by flow cytometry, using a fluorescein isothiocyanate-conjugated antibody and the P-gp fluorescent subtract rhodamine 123, respectively. A significant increase in P-gp expression was observed as soon as 6 h after exposure to 5 μM doxorubicin. P-gp activity also increased after 6 h, but only at higher doxorubicin concentrations (over 50 μM). Paraquat (0-5,000 μM) cytotoxicity was then evaluated with or without previous exposure of the cells to doxorubicin (5-100 μM, a concentration range causing both an increase in P-gp expression and activity). Under P-gp induction, a significant reduction in paraquat cytotoxicity was observed. Furthermore, when these cells were incubated with a specific P-gp inhibitor (UIC2 antibody) the doxorubicin protective effects were blocked, confirming the involvement of P-gp in the reduction in paraquat cytotoxicity. In conclusion, the human Caco-2 cell line model can be used for the study of P-gp induction as an antidotal pathway against substrates of this transporter system.