Organophosphates are known primarily as neurotoxins. However, reactive oxygen species (ROS) caused by organophosphates may be involved in the toxicity of various pesticides. Therefore, in this study we aimed to examine how an organophosphate insecticide, chlorpyrifos-ethyl (CE) [0,0-diethyl 0 (3,5,6-trichloro-2-pyridyl) phosphorothioate], affects lipid peroxidation and the antioxidant defense system in vitro. For this purpose, four experiments were carried out. In experiment I, erythrocyte packets obtained from six (three male, three female) volunteers were divided into six portions, and to each was added CE in both a high concentration range (0, 0.4, 2, 10, 50, 100 g/l) and a low concentration range (0, 0.01, 0.1 g/l). Additionally, each concentration group was divided into five tubes, and incubated at +4 degreesC for 0, 30, 60, 120, and 240 min. After incubation, the levels of malondialdehyde (MDA) and the activity of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) were determined in the erythrocytes in all tubes. In experiment 2, to examine the effect of CE (or its main metabolites) on the activity of purified, commercially available enzymes, CE at concentrations of 0, 0.01, 0.1, 0.4, and 10 g/l was incubated with purified SOD, GSH-Px and CAT at the concentrations observed in control group at the 0 CE concentration level in experiment 1 for 1 h at room temperature (25 degreesC). In experiment 3, the xanthine-xanthine oxidase system was used to determine whether the activities of SOD, GSH-Px and CAT were inactivated other than by CE, for example by superoxide radicals inducing lipid peroxidation in erythrocytes. Samples with xanthine and xanthine oxidase were mixed and incubated for 1 h at room temperature (25 degreesC). In experiment 4, to determine whether enzyme activities were still inhibited if lipid peroxidation was prevented by exogenous antioxidants, experiment 1 was repeated with the CE concentrations of 0.01, 0.1, 0.4, and 10 g/l by adding butylated hydroxytoluene and vitamin E to the medium. The MDA levels were determined spectrophotometrically. Enzymatic methods were used for the determination of SOD, GSH-Px, and CAT activities. The Friedman test and Wilcoxon's Signed Ranks test were used to compare paired groups. MDA values and GSH-fx activities increased with increasing CE concentration and incubation period (P < 0.05), but SOD and CAT activities decreased with increasing CE concentration and incubation period (P < 0.01). From these results, it can be concluded that in vitro administration of CE resulted in the induction of erythrocyte lipid peroxidation and significant changes in antioxidant enzyme activities, suggesting that ROS and/or free radicals may be involved in the toxic effects of CE.