This study aims to investigate the effects of the herbicide 2,4-D and the insecticide azinphosmethyl on hepatic antioxidant enzyme activities and lipid peroxidation in tilapia. Fish were exposed to 27 ppm 2,4-D, 0.03 ppm azinphosmethyl and to a mixture of both for 24, 48, 72 and 96 h. Activities of catalase (EC 126.96.36.199), glutathione-S-transferase (GST, EC 188.8.131.52) and the level of malondialdehyde (MDA) in the liver of Oreochromis niloticus exposed to 2,4-D and azinphosmethyl, both individually and in combination, were not affected by the pesticide exposures. However, glucose-6-phosphate dehydrogenase (G6PD, EC 184.108.40.206) and glutathione reductase (GR, EC 220.127.116.11) activities in individual and combined treatments, increased significantly compared to controls. Furthermore, glutathione peroxidase (GPx, EC 18.104.22.168) activity increased in individual treatment, while the same enzyme activity decreased in combination. 2,4-D did not affect the activity of superoxide dismutase (SOD, EC 22.214.171.124), but the activity of this enzyme in azinphosmethyl treatment decreased, while its activity increased in combination. Combined treatment of the pesticides exerted synergistic effects in the activity of SOD, while antagonistic effects were found in the activities of G6PD, GPx, GR. The results indicate that O, niloticus resisted oxidative stress by antioxidant mechanisms and prevented increases in lipid peroxidation. (C) 2000 Elsevier Science Inc. All rights reserved.