Numerous reports have demonstrated that oxidative stress induced by diabetes plays an important role in the development and progression of diabetic vascular complications including nephropathy. Indeed, there is emerging evidence that the formation of reactive oxygen species (ROS) is a direct consequence of hyperglycemia. Biomarkers for oxidative damage to DNA, lipids, and proteins are also supporting the concept of increased oxidative stress in diabetes and diabetic nephropathy. However, there is an unanswered question: When does oxidative stress as a pathogenetic event occur in the process of diabetic nephropathy? To answer this question, glomerular ROS was imaged with the use of 2', 7'-dichlorofluorescein diacetate (DCFH-DA). The image of DCF fluorescence was strong in glomeruli from diabetic rats as compared with that of glomeruli from nondiabetic control rats. mRNA expression of antioxidant enzymes such as catalase, glutathione peroxidase, Cu/Zn superoxide dismutase, and heme oxygenase-1 (HO-1) was also determined because oxidative stress definitely refers to the situation of an imbalance between the production of ROS and antioxidant defense. The mRNA expression of catalase, glutathione peroxidase, and Cu/Zn superoxide dismutase 2 wk after the induction of diabetes was not significantly different from that in control rats. Alternatively, mRNA and protein expression of HO-1 was strongly induced by 16-fold in diabetic glomeruli after the induction of diabetes. Antioxidant treatment with either vitamin E or probucol almost completely normalized HO-1 overexpression in diabetic glomeruli, supporting the existence of oxidative stress in the glomeruli of early diabetes. Furthermore, It has reported that antioxidant treatment with vitamin E, probucol, alpha-lipoic acid, or taurine normalized diabetes-induced not only renal dysfunction such as albuminuria and glomerular hypertension but also glomerular pathologies. In summary, oxidative stress by diabetes could play a crucial role in the development and progression of diabetic nephropathy, and antioxidant treatment could be a potential therapeutic procedure for diabetic nephropathy.