Erythrocytes are a convenient model to understand the membrane oxidative damage induced by various xenobiotic pro-oxidants. This study was designed to investigate the possibility of methomyl (Lannate 90% SP), S-methyl N-(methylcarbamoyloxy) thioacetimidate, to induce oxidative stress response in rat erythrocytes in vitro. Erythrocytes were incubated for 4 hours at 37 degrees C with different concentrations (0.0, 0.1, 0.5, 1.0, 1.5 and 2.0 mM) of methomyl. The results showed that methomyl decreased acetylcholinesterase (AChE), superoxide dismutase (SOD) and glutathione S-transferase (GST) activities and increased level of lipid peroxidation (LPO) as well as the percentage of haemolysis. The response occurred in a concentration-dependent manner. The study suggested that methomyl has the capability to induce oxidative damage as evidenced by increasing LPO and perturbations in various antioxidant enzymes.