Little is known of the interactions between diabetes and sex hormones on vascular function. The objectives of this study were to investigate whether there were sex differences in rat aortic endothelial function one week after the induction of streptozotocin (STZ)-diabetes, and to examine the potential roles of superoxide and nitric oxide (NO) in this sex-specific effect. Endothelium-dependent vasodilatation to acetylcholine (ACh) was measured in rat aortic rings before and after treatment with MnTMPyP (25 μM), a superoxide dismutase. Contractile responses to phenylephrine (PE) were generated before and after treatment with l-NAME (200 μM), a nitric oxide synthase (NOS) inhibitor. The mRNA expression of NADPH oxidase (Nox) and endothelial nitric oxide synthase (eNOS) were also determined. We demonstrated that (1) STZ-diabetes impaired endothelium-dependent vasodilatation to ACh to a greater extent in female than male aortae, (2) inhibition of superoxide enhanced sensitivity to ACh only in diabetic females, and (3) Nox1 and Nox4 mRNA expression were significantly elevated only in aortic tissue of diabetic females. Furthermore, incubation of aortic rings with l-NAME potentiated PE responses in all groups, but aortae from control females showed a greater potentiation of the PE response after NOS inhibition compared with others. STZ-diabetes reduced the extent of PE potentiation after l-NAME and the aortic eNOS mRNA expression in females to the same levels as seen in males. These data suggest that a decrease in NO, resulting from either decreased eNOS or elevated superoxide, may partially contribute to the predisposition of the female aorta to injury early in diabetes. © 2013 Elsevier B.V.