The effects of etoxazole were evaluated in freshwater fish Oreochromis niloticus from five different sublethal etoxazole concentrations in order to study the biochemical response, photometrically. No changes were observed in the activities of antioxidant enzymes such as superoxide dismutase (SOD, EC 188.8.131.52), catalase (CAT, EC 184.108.40.206), and glutathione peroxidase (GPx, EC 220.127.116.11). These were measured in liver after 1, 7, and 15 days of exposure to sublethal concentrations of 0.2, 0.4, 0.6, 0.8, and 1% of field application rate (134.75 ppm). This study also investigated the levels of neurotoxic effects by the determination of acetylcholine esterase (AChE EC 18.104.22.168) and sodium-potassium adenosine 5'-triphosphatase (Na+K+-ATPase, EC 22.214.171.124) activities. The exposure of fish led to sharp depletion in AChE activity while there is no significant alteration in Na+K+-ATPase activity. Up to 80% decreases were observed in the AChE activity. Since no difference was found in the activity of glutamate-pyruvate transaminase (GPT, EC 126.96.36.199), etoxazole did not show hepatotoxic effect. The results of the present study show that increase in the malondialdehyde (MDA) contents and decrease in the AChE activity can be used as biomarkers for monitoring toxicity in etoxazole exposure. (C) 2003 Elsevier Inc. All rights reserved.