This study was carried out to examine a part of the mechanism for the etiology of diabetic complications. Thirty normal and forty streptozotocin(STZ)-induced diabetic rats were used as the animal models. The animals were sacrificed at the time points of 3 days, 1,2,4 and 6 weeks after STZ-injection and a time course changes in the concentrations of thiobarbituric acid-reactive substances(TBARS) in blood, urine, and tissues, along with the levels of conjugated dienes in tissues were measured as indices of in vivo lipid peroxidation. The activities of antioxidant enzymes, catalase, superoxide dismutase, glutathione peroxidase and the levels of blood retinol and alpha-tocopherol were also measured. The diabetic rats maintained a slightly higher plasma TBARS level throughout the experiment. The urinary TBARS level was significantly higher in diabetic group and gradually increased with time. Concentrations of TBARS in liver, heart, and kidney tissues from diabetic animals were higher than those from the normal group. An increase of conjugated dienes was also observed in the all tissues examined. The kidney tissue of diabetic animals revealed more significant lipid peroxidation state than any other organ tissues. The activities of hepatic antioxidant enzymes such as catalase, superoxide dismutase, glutathione peroxidase were higher in diabetic animals compared to the control ones and increased with the duration of diabetes mellitus. The plasma levels of vitamin A and E were loser in diabetic animals than in normal controls throughout the experimental period. The level of vitamin E in diabetic animals was significantly decreased with the duration of the disease. The results of this study suggest that an effective regimen to suppress the adverse changes in lipid peroxidation and antioxidant defense system is required from the early stage of the disease to prevent the development of diabetic complications. (Korean J Nutrition 34(3) : 253∼264, 2001)