Species- and tissue-specific defenses against the possibility of oxidative stress and lipid peroxidation were compared in adult fish, Oreochromis niloticus and Cyprinus carpio, exposed to 2,4-dichlorophenoxyacetic acid (2,4-D), azinphosmethyl and their combination for 96 h. Superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase activities were monitored in kidney, brain and gill. In all exposure groups there was a marked increase in SOD activity in gill tissues in both fish species, while it was at the control level in other tissues. The highest elevation of SOD activity by combined treatment was observed in C carpio. Individual and combined treatments caused an elevation in catalase and GPx activities in kidney of C carpio. Catalase activity was unaffected in brain of O. niloticus, while GPx activity was decreased after all treatments. Glutathione S-transferase (GST) activity was higher than the control levels in kidney of both fish exposed to pesticides. No significant changes were observed in malondialdehyde level in kidney and brain of C. carpio. Our results indicate that the toxicities of azinphosmethyl and 2,4-D may be related to oxidative stress. Also, the results show that SOD activity in gill and GST activity in kidney may be used as biomarkers for pollution monitoring and indicate that the activities of certain biomarkers in C. carpio are more sensitive to pesticides than those in O. niloticus. (C) 2003 Elsevier Inc. All rights reserved.