The mature male Wistar rats (n = 36, 300-320 g) were divided into six groups having six animals, i.e., Group I (control), Group II (catechin, 20 mg/kg bw), Group III (quercetin, 20 mg/kg bw), Group IV (chlorpyrifos 5.4 mg/kg, 1/25 LD50), Group V (catechin + chlorpyrifos) and Group VI (quercetin + chlorpyrifos). Rats were given chlorpyrifos, catechin and quercetin daily via gavage for 4 weeks. Chlorpyrifos increased the levels of malondialdehyde (MDA), superoxide dismutase (SOD), and catalase (CAT), and decreased glutathione peroxidase (GPx) and glutathione-S-transferase (GST) activities in lung tissues compared to the control group. In the catechin plus chlorpyrifos- and quercetin plus chlorpyrifos-treated groups, there were statistically significant increases in CAT and SOD activities, while no statistically significant changes were observed in MDA, GST and GPx activities relative to the control. Compared to the chlorpyrifos-treated group, however, the catechin plus chlorpyrifos- and quercetin plus chlorpyrifos-treated groups showed significantly increased GST and GPx activity, while the activity of MDA, SOD and CAT was significantly decreased. Light microscopic investigations revealed that 4 weeks of chlorpyrifos exposure induced numerous histopathological alterations in the lung. Milder histopathological changes were observed in animals co-treated with catechin plus chlorpyrifos or quercetin plus chlorpyrifos. Thus, it appears that catechin and quercetin ameliorate chlorpyrifos-induced lung toxicity but are not completely protective. (C) 2010 Elsevier Ltd. All rights reserved.