Oxidative stress has been implicated in the pathogenesis of bleomycin-induced lung fibrosis and many antioxidant agents have been studied for prevention and treatment of the disease in animals and humans. We therefore examined whether Ginkgo biloba (Gb), a flavonoid-rich antioxidant, inhibits bleomycin-induced lung fibrosis in rats. Male Sprague-Dawley rats were given a single dose of bleomycin (2.5 mg/kg, intratracheally) in pulmonary fibrosis groups and saline in controls. First dose of Gb was given a day before the bleomycin injection and continued until sacrifice. At day 14, fibrotic changes in lung were estimated to occur by Aschoft's criteria and lung hydroxyproline content. Bleomycin challenge provoked severe pulmonary fibrosis with marked increase in hydroxyproline content of lung tissue and typical histological findings, which is prevented by Gb. Hydroxyproline level was significantly higher (13.51+/-0.87 mg/g dried tissue) in bleomycin treated rats than controls (9.2+/-1.33), and its level was remained to the control levels (7.38+/-0.76) in rats treated with prophylactic Gb. On the other hand, bleomycin injection significantly reduced activities of glutathione peroxidase, superoxide dismutase and catalase in lung tissue which is prevented by Gb. Also, bleomycin injection resulted in a marked increase of malondialdehyde and nitrite level which is attenuated by Gb. The data suggest that Gb has a potent antioxidant activity in the model of bleomycin-induced lung fibrosis in rats, and therefore has a potent antifibrotic activity against bleomycin-induced lung fibrosis model in rats.