Organophosphate pesticides are known to induce oxidative stress and cause oxidative tissue damage, as has been reported in studies concerning acute and chronic intoxication with these compounds. Our objective was to investigate the activities of brain antioxidant enzymes and malonyldialdehyde, as well as the level of carbonyl groups, in rats sub-chronically intoxicated with chlorpyrifos at doses of 0.2, 2 and 5 mg per kg of body weight per day. It was found that chlorpyrifos induces change in brain antioxidant enzymes, such as superoxide dismutase, catalase and glutathione peroxidise, but to a different degree in comparison to proper control values; however, the elevated antioxidant enzymes activities failed to check lipid and protein peroxidation in the brains of rats. Thus, in sub-chronic intoxication with chlorpyrifos, as evidenced by increased level of malonyldialdehyde and carbonyl groups, oxidative stress is induced. Measurements of protein carbonyl groups appeared to give more consistent responses in the rats’ brains when compared to the malonyldialdehyde level after sub-chronic chlorpyrifos treatment.