Antioxidant therapy may be useful in diseases with impaired oxidant antioxidant balance such as lung fibrosis. The effects of sulfhydryl-containing antioxidant agents N-acetylcysteine (NAC) and erdosteine on the bleomycin-induced lung fibrosis were compared in rats. The animals were divided into four groups: Vehicle + vehicle, vehicle + bleomycin (2.5 U/kg), bleomycin + (10 mg/kg), and bleomycin + NAC (3 mmol/kg). Bleomycin administration resulted in prominent lung fibrosis as measured by lung hydroxyproline content and lung histology which is almost completely prevented by erdosteine and NAC. Hydroxyproline content was 18.7 +/- 3.5 and 11.2 +/- 0.6 mg/g dried tissue in bleomycin and saline treated rats, respectively (P < 0.001), and this level was 11.3 +/- 1.2 and 13.8 +/- 1.2 mg/g dried tissue in erdosteine and NAC pretreated, respectively. Erdosteine and NAC significantly reduced depletion of glutathione peroxidase, and prevented increases in myeloperoxidase activities, nitric oxide, and malondialdehyde levels in lung tissue produced by bleomycin. Data presented here indicate that erdosteine and NAC similarly prevented bleomycin-induced lung fibrosis and their antioxidant effects were also similar in this experiment.