Little is known about the interaction between diabetes and sex in vasculature. This study was designed to investigate whether there were sex differences in rat aortic endothelium-dependent vasodilation (EDV) in Zucker diabetic fatty (ZDF) rats, and the potential role of superoxide. EDV to acetylcholine (ACh) was measured in aortic rings pre-contracted with phenylephrine before and after pretreatment with apocynin (100 ?M), a NADPH oxidase (Nox) inhibitor. In addition, the level of Nox (a potent source of superoxide) and PKC? mRNA expression were determined using real-time RT-PCR. ACh-induced relaxations were significantly greater in female lean rats compared with male lean rats. Accordingly, male lean rats had higher PKC?I expression level than female lean rats. Diabetes significantly impaired EDV in aortic rings from female ZDF rats, however, potentiated the relaxation in males. Pre-incubation of aortic rings with apocynin increased EDV only in diabetic female group, suggesting impairment of EDV in female ZDF aorta was partly due to an increase in the activity of superoxide. Accordingly, the levels of Nox1, Nox4, and PKC? mRNA expression were substantially enhanced in aorta of female ZDF rats compared to those in lean animals. These data suggest that an elevation of superoxide may partially contribute to the predisposition of the female aorta to injury in type 2 diabetes. Grant Funding Source: NIH/NIDCR.