Insulin resistance has been shown to be associated with increased blood pressure (BP). The sex hormones estrogen and testosterone have opposing effects in the development of increased BP. Since testosterone has been implicated in increased BP following insulin resistance, we have tried to dissect out the effects of insulin resistance on endothelium-dependent vasorelaxation in the presence and absence of testosterone. Both gonadectomized and sham-operated male Wistar rats fed with a high fructose diet developed insulin resistance, but BP increased only in the sham-operated rats. Reintroduction of testosterone in vivo restored the increase in BP thereby abolishing the protective effects of gonadectomy. Fructose feeding did not affect plasma testosterone levels. Insulin resistance induced endothelial dysfunction in the mesenteric arteries of sham-operated rats, which was prevented by gonadectomy, thus suggesting a key role for testosterone in the pathogenesis of secondary vascular complications. Subsequent to blocking the actions of endothelium dependent hyperpolarizing factor (EDHF), relaxation to acetylcholine (ACh) was lower in sham-operated fructose-fed rats compared to other groups, suggesting the involvement of nitric oxide (NO) in vasorelaxation. Inhibition of NO-synthesis nearly abolished the ACh-evoked relaxation in both fructose-fed groups thus suggesting a testosterone-independent impairment of EDHF-mediated relaxation. The improvement in endothelial function following gonadectomy could be ascribed to a NO component, although plasma nitrite and nitrate levels were unchanged. In summary, testosterone is essential in vivo for the development of endothelial dysfunction and hypertension secondary to insulin resistance suggesting a facilitatory role for testosterone in increasing BP in fructose-fed male rats.