Paraquat (PQ) is a herbicide that is very toxic to all living organisms. It generates free radicals and leads to acute or chronic lung injury. Free radicals are often associated with fibrogenesis, which occurs in various disease states. The purpose of this study was to determine whether captopril prevents paraquat toxicity in lung tissue. Paraquat alone increased the level of lipid peroxidation (LPO) and the activity of superoxide dismutase (SOD) after 4, 12, 24 and 72 h of administration. Also, the level of hydroxyproline showed an increase after 24 h of paraquat administration. However, paraquat also decreased the level of glutathione (GSH) and the activity of glutathione peroxidase (GSH-Px). Captopril (50 mg/kg i.p.) and paraquat were simultaneously injected (40 mg/kg i.p.), and the captopril injection 1 h after paraquat ameliorated the biochemical toxicity induced by paraquat. This was evidenced by a significant reduction in LPO and balancing the endogenous antioxidant capacity by normalizing the activities of SOD and GSH-Px and the GSH content in the lung tissue. Moreover, captopril injection prevented the increase of hydroxyproline content as an index of lung fibrosis. From these results, the beneficial effects of captopril on paraquat toxicity appear to be through enhancement of the endogenous antioxidant system preventing the lung fibrosis.