Toxicity of organophosphorus insecticides is mainly due to the inhibition of acetylcholinesterase, but, oxidative stress may be involved in the toxicity of this pesticides. Therefore, it was investigated whether diazinon, a commonly used organophosphate, may induce oxidative stress and cholinesterase inhibition in different tissues of Cyprinus carpio. Sublethal concentrations of diazinon (0.0036, 0.018 and 0.036 ppb) were administired to C. carpio L. for 5, 15 and 30 days. The study was made by measuring biochemical stress responses of C. carpio L. spectrophotometrically taking into account acetylcholinesterase (AChE), Na+K+-adenosine triphosphatase (Na+K+-ATPase) and other antioxidant enzyme activities, as well as malondialdehyde and protein contents in gill, muscle and kidney tissues of the fish. Results of the study suggest that AChE (in gill and muscle tissues) and Na+K+-ATPase (in muscle and kidney tissues) activities decreased; that antioxidant enzymes, in particular superoxide dismutase (SOD), increased in gill, kidney and muscle tissues. We also observed the existence of a protective function of antioxidant enzymes against lipid peroxidation in muscle tissue. The changes in NIDA content varied between increases and decreases in kidney tissue. In gill tissue, however, lipid peroxidation could not be prevented despite induction of SOD and glutathione peroxidase activities. We could see that the protein content decreased only in gill tissue as diazinon dosage was gradually increased until the 15th day of the experiment. During the period between 15th and the 30th days, the protein level in the fish was observed to have reached to that of the control group. This change in protein level can be attributed to adjustment of the fish to its new environmental conditions. Considering most of the parameters in tissues, it can be stated that diazinon exerted its effect at low concentration and during a long period of time, and its toxicity increased dose dependently. This study reveals that C. carpio developed tissue-specific adaptive response to neutralize the oxidative stress following pesticide exposure depending on different antioxidant levels in tissues and that SOD can be used as a biomarker in determining diazinon toxicity due to its early response at even low concentration levels (c) 2006 Elsevier B.V. All rights reserved.