Background The most intimidatory pathological changes in patients with DM are cardiovascular illnesses, which are the major causes of death in diabetic patients and are far more prevalent than in nondiabetics because of accelerated atherosclerosis. In this study, we tried to clarify the changes in macrovascular endothelial ultrastructure and in the gene expression of endothelial nitric oxide synthase (eNOS)mRNA in diabetic rats. Methods The study was conducted on 52 of 10-week old Sprague Dawley (SD) rats with body weight of (320±42) g. SD rats were divided into: experimental group treated with a single intraperitoneal injection of streptozotocin (STZ, 60 mg/kg), (male, n=20, diabetes mellitus (DMM)); female, n=12, diabetes mellitus female (DMF)) and control group (male, n=10, diabetes mellitus male control (DMMC); female, n=10, diabetes mellitus female control (DMFC)). Four weeks after treatment, half of the rats were sacrificed; the remainders were sacrificed ten weeks after treatment. One part of the abdominal aortic sample was stored under glutaraldehyde (volume fraction ψB = 2.5 %). After the process of chemical fixation, chemical dehydration, drying and conductivity enhancement, all samples were observed and photographed using scanning electron microscopy (Leica-Stereoscan 260, England). The other part of the abdominal aortic sample was treated with liquid nitrogen and the expression of eNOSmRNA was assessed by semi-quantitative RT-PCR. Results The aortic lumen of both experimental groups adsorbed much more debris than that of either control group. The endothelial surfaces of diabetic rats were coarse, wrinkled and protuberant like fingers or villi. The vascular endothelial lesions of diabetic male rats were very distinct after 4 weeks, and as obvious as those at 10 weeks. The vascular endothelial lesions of diabetic female rats were not severe at 4 weeks and only became marked after 10 weeks. In both males and females, the abdominal aortic eNOSmRNA content of 4 weeks and 10 weeks diabetic rats was very significantly lower (P