The aim of this study was to investigate the effects of organophosphorus (OP) pesticide diazinon on acetycholinesterase (AChE: EC 126.96.36.199) activity and its relationship to lipid peroxidation (LPO) in the brain of a freshwater fish, Oreochromis niloticus. Malondialdehyde (MDA) content was used as biomarker for LPO. Fish were exposed to 1 and 2 mg/L sublethal concentrations of diazinon for 1, 7, 15 and, 30 days. In the entire experimental group, AChE activity in brain significantly decreased (up to 93% of control), whereas MDA content decreased after 1 day, and increased after 7 and 15 days of exposures. MDA was in similar level with the control group after diazinon exposure of 30 days. The findings of the present study show that diazinon inhibited AChE activity and it has LPO-inducing potential in fish. The inhibition of AChE activity in the brain of O. niloticus correlated with increased MDA levels after 7 and 15 days diazinon exposures (r= -0.66 1, P < 0.0 19; r = -0.652, P < 0.022, respectively). (c) 2005 Elsevier B.V. All rights reserved.