Several studies suggest that xanthine dehydrogenase (XDH) and its oxidase form (XO) play an important role in various types of ischemic and vascular injuries. Recently, we have demonstrated that estradiol (E2) induces a significant decrease of the expression and activity of XDH and of its conversion to XO in human mammary epithelial cells. E2 is known to induce upregulation of eNOS gene expression in aortic endothelial cells. Because the XO-derived O2•– combines with •NO to yield ONOO–, and considering that ONOO– converts XDH to XO, the resulting increase of XO activity and reactive oxygen species production would eventually lead to a further increase of ONOO– production, thus creating a vicious cycle of oxidative stress. Our previous study has indicated that sildenafil has a protective effect on human mammary epithelial cells as a consequence of XO inhibition and of the resulting decrease of free oxygen radicals that can impair the expression of NADPH oxidase and type 5 phosphodiesterase (PDE-5). In the present study, we report that the dual inhibitory effect exerted by sildenafil on both XO and PDE-5 is a consequence of a structural modification induced by O2•–, also consisting of the release of a piperazine group that could in turn inhibit the XO enzyme.