Background:Reports have clearly indicated the role of oxidative stress in the pathogenesis of organophosphate pesticides (Op) toxicity. However, there is dearth of information on which group of the farm workers is more at risk of Op-induced oxidative stress.Aim:This study determined serum levels of malondialdehyde (MDA), catalase (CAT), glutathione peroxidase (GPx), reduced glutathione (GSH), myeloperoxidase (MPO), nitric oxide (NO), and serum activity of acetylcholinesterase (AChE) in farm workers exposed to Op.Subjects and Methods:A total of 60 (30 pesticide applicators and 30 farmers) and 30 apparently healthy non-farmers who were nonexposed to Op (controls) were recruited into this study. Serum activity of AChE was determined using high performance liquid chromatography (HPLC), while serum levels of MDA, GSH, and NO and serum activities of CAT, MPO, GPx, and superoxide dismutase (SOD) were determined colorimetrically.Results:Serum activities of AChE and CAT were significantly lower, whereas MPO activity was significantly higher in pesticide applicators compared with controls. Similarly, farmers had significantly reduced serum AChE activity and significantly raised MPO activity compared with controls. However, serum activities of AChE, CAT, and MPO were significantly lower, whereas mean level of MDA was significantly higher in pesticide applicators compared with farmers.Conclusion:This study shows that Op applicators are more exposed to oxidative stress than farmers, thus Op applicators require increased antioxidant supplements than farmers.