Abstract: Synthetic pyrethroids, such as deltamethrin, are toxic substances that lead to generation of reactive oxygen species, which harm living organisms. The present study deals with the level of imbalance evolved by sub-lethal concentrations of deltamethrin on the antioxidant enzyme catalase (CAT), lipid peroxidation (LPO) and the reduced glutathione (GSH) of Danio rerio brain and muscle during 16-days exposure period. For this study, matured adult fishes were exposed to different concentrations viz., 0.016, 0.025 and 0.043 μg L-1 (96-h LC5, LC10 and LC20 of deltamethrin) for 16 days continuously. In the brain and muscle of fish an inhibition of catalase (CAT) activity was observed while for lipid peroxidation (LPO) a continuous enhancement was apparent for all exposure periods. GSH level in both the tissues were decreased for all exposure periods. The toxicity was time as well as concentration dependent. However, there is much to learn about the details of this phenomenon and further research is needed to fully elucidate the effect of pesticides and the environmental risks they pose in the aquatic organisms.