Little is known of the interaction between diabetes and gender in the vasculature. This study was designed to investigate 1) whether there were gender differences in rat aortic endothelium-dependent vasodilation (EDV) in streptozotocin (STZ, 60 mg/kg, iv)-induced diabetic rats at early stage of the disease (1 wk), and 2) the potential role of superoxide in diabetes-induced vascular dysfunction. EDV to acetylcholine (ACh; 10?8 to 10?5 M) was measured in aortic rings precontracted with phenylephrine (2 ?M) before and after pretreatment with MnTMPyP (10 mM), a superoxide scavenger. In addition, the level of endothelial nitric oxide synthase (eNOS) and NADPH oxidase (Nox, a potent sources of superoxide) 1, 2 and 4 mRNA expression were determined using real-time RT-PCR. ACh-induced relaxations were significantly impaired in aortic rings from both male and female diabetic rats. However, the extent of impairment was significantly greater in diabetic females than diabetic males. Preincubation with MnTMPyP increased EDV only in diabetic female group. Accordingly, our data showed that in females, the level of Nox4 mRNA expression was substantially enhanced, whereas the level of eNOS mRNA expression was decreased at 1 wk after the induction of diabetes. These data suggest that the predisposition of female rat aorta to vascular injury in diabetes is possibly due to the superoxide production (supported by NIH/NIDCR).