Paraquat as a bipyridyl compound is widely used as an effective herbicide worldwide. In this study, oxidative stress was investigated in blood samples of workers in a pesticide factory, formulating paraquat products for use in agriculture. Controls were age-matched workers with no history of pesticide exposure. They were measured for lipid peroxidation (LPO), antioxidant power and total thiol (SH) groups in blood. The results expressed as mean+/-SD show induction of oxidative stress in workers as revealed by increased plasma LPO (11.46+/-0.99 vs 10.11+/-0.69, P<0.001), decreased plasma antioxidant capacity (1.35+/-0.03 vs 1.54+/-0.05, P<0.001) and plasma SH groups (0.16+/-0.01 vs 0.21+/-0.01, P<0.001) in comparison to those of controls. It is concluded that paraquat-formulating factory workers have elevated LPO and decreased antioxidant power, which may put them in further consequences of oxidative stress.