Abstract: The effect of chronic exposure to menazon (Organophosphorus compound) in the production of oxidative stress was assessed in rats. Administration of menazon (50, 100, 500, 1000 ppm) for 2 weeks duration increased catalase (CAT), superoxide dismutase (SOD) activities in red blood cells (RBC). However, acetylcholinesterase (AChE) activity were decreased in these samples. The increase in RBC lipid peroxidation correlated well with the inhibition in RBC AChE activity. Increased activities of CAT and SOD showed significant correlations in RBC samples when different doses of menazon were used. The results of the present study suggest the usefulness of RBC AChE measurement as a good index in the evaluation of menazon-induced oxidative stress affecting blood.