Organophosphorus compounds may induce oxidative stress leading to generation of free radicals and alterations in antioxidant and scavengers of oxygen free radicals (OFRs). The effect of subchronic exposure to malathion in the production of oxidative stress was evaluated in male Wistar rats. Administration of malathion (100, 316, 1000, 1500 ppm) for 4 weeks increased catalase (CAT), superoxide dismutase (SOD) activities as well as malondialdehyde (MDA) concentration in red blood cells (RBC) and liver. However, acetylcholinesterase (AChE) and cholinesterase (ChE) activities were decreased in these samples. The increase in RBC and liver lipid peroxidation correlated well with the inhibition in RBC AChE and liver ChE activities. Elevation of MDA concentrations and increased activities of CAT and SOD showed significant correlations in both RBC and liver samples when different doses of malathion were used. The results of the present study suggest the usefulness of RBC AChE measurement as a good biomarker in the estimation of malathion-induced oxidative stress affecting blood and liver.