Insecticides have potential to non-target organisms, disrupting the healthy functioning of the aquatic environment as they are the ultimate receptor of the aquatic ecosystem. Insecticides, which are widely used in agriculture, have high neurotoxicity on aquatic organisms. In this study, the acute alterations [catalase (CAT), arylesterase (ARE), malondialdehyde (MDA), myeleperoxidase (MPO), paraoxonase (PON), glutathione peroxidase (GPX), superoxide dismutase (SOD), 8-hydroxy-2-deoxyguanosine (8-OHdG) level, caspase-3 activity, and Acetylcholinesterase (AChE) enzyme activity] caused by the different concentrations of Fipronil (FP) insecticide (0.05, 0.1, and 0.2 mg/L) on rainbow trout (Oncorhynchus mykiss) brain tissue were investigated. It has been determined that superoxide dismutase -catalase - glutathione peroxidase - paraoxonase and arylesterase enzyme activities were inhibited but MDA and MPO induced depending on the concentration in brain tissue. When compared with the control group, the changes between the pesticide exposed groups were found statistically significant (p < 0.05). In brain tissue, while AChE enzyme activity was decreased depending on concentration, caspase-3 activity increased with 8-OHdG level. As a result, it has been determined that FP is a dangerous environmental pollutant for aquatic organisms, even at low concentrations, inducing oxidative stress, damaging the brain tissue of fish and stimulating apoptosis.