en Biochemistry Biochemistry پژوهشي Orginal research papers Chlorpyrifos (CPF) is used intensively as an insecticide. There is a high risk of interference with the aquatic environment due to unconscious use and has a negative effects especially fish. In this study, the responses of rainbow trout (<em>Oncorhynchus mykiss</em>) and brown trout (<em>Salmo trutta fario</em>) exposed to the same pollutants have been evaluated and compared in terms of target organs. Fish were exposed to different concentrations of CPF (0.25, 0.5, and 1 &micro;g L^-1) through 21 days. After the process oxidative stress [superoxide dismutase (SOD), catalase (CAT glucose-6-phosphate dehydrogenase (G6PD), glutathione reductase (GR), glutathione-S-transferase (GST), glutathione peroxidase (GPx)], acetylcholinesterase (AChE) and malondialdehyde (MDA) have been measured in gill, kidney and liver tissues. CPF exposure led to a significant changes in the enzyme activities and decreased AChE in all tissues (<em>p</em><0.05). All antioxidant enzyme activities and MDA levels showed tissue-specific alterations (<em>p</em><0.05). These results put forward a close relationship between AChE inhibition and chlorpyrifos concentrations. <a>In addition to</a>&nbsp;, CPF concentrations &nbsp;caused oxidative stress and inhibition in all enzyme activities of two different trout species&rsquo; gill, liver and kidney tissues. <div> <hr align="left" size="1" width="33%" > <div><div id="_com_1" uage="JavaScript"></div></div> </div> AChE, Chlorpyrifos, Fish, Oxidative stress responses, Pesticide 2106 2117 http://jifro.ir/browse.php?a_code=A-10-2701-1&slc_lang=en&sid=1 A. Uçar arzuucar@atauni.edu.tr 100319475328460035750 100319475328460035750 Yes facutly of fisheries V. Parlak veysel.parlak@atauni.edu.tr 100319475328460035751 100319475328460035751 No facutly of fisheries G. Alak galak@atauni.edu.tr 100319475328460035752 100319475328460035752 No facutly of fisheries M. Atamanalp mataman@atauni.edu.tr 100319475328460035753 100319475328460035753 No facutly of fisheries M. Şişecioğlu 100319475328460035754 100319475328460035754 No facutly of fisheries