The mechanisms of free-radical injury include reactions with proteins, nucleic acids, and polysaccharides; and covalent binding to membrane components and initiation of lipid peroxidation. Cells have developed antioxidant defense to prevent free-radical injury including superoxide dismutase (SOD) and glutathione peroxidase (GPx). Significantly higher concentrations of total malondialdehyde (MDA) in plasma (1.22±0.42 vs. 0.64±0.22μimol/L, p<0.0001) as well as erythrocytes (2.56±1.28 vs. 1.03±0.44μmol/L, p<0.0001) of the CAPD patients were found when compared to the control group. The free MDA in plasma and the erythrocytes do not differ significantly in continuous ambulatory peritoneal dialysis (CAPD) patients and the control group. A significantly lower activity of GPx in erythrocytes of CAPD patients (17.85±2.63 U/g Hb vs. 23.26±3.61 U/g Hb, p<0.0001) was found when compdred to the control group, but the SOD activity in erythrocytes is not different (2272.36±579.92 U/g Hb vs. 2347.13±502.51 U/g Hg, NS). Our results show an increase of total MDA in erythrocytes and plasma. MDA is the product of lipid peroxidation with decreasing activity of GPx, which is capable of detoxifying peroxides. The activity of SOD did not change in CAPD patients. These results propose a possible role of free radicals with reduced antioxidant activity of GPx in CAPD patients and indicate that they could play some role in other pathological conditions such as atherogenesis and hemolysis.