This study examines the effect of male sex hormones on the release, metabolism and function of endothelial nitric oxide (eNO) in rat aorta. Aortic segments from orchidectomized and control male Sprague-Dawley rats were used to measure eNO synthase (eNOS) expression, nitric oxide (NO) release, acetylcholine (ACh)- and sodium nitroprusside (SNP)-induced relaxation and Cu/Zn-superoxide dismutase (SOD) expression and activity. eNOS expression as well as basal and ACh-induced NO release were similar in arteries from both groups of rats. Basal superoxide anion production was similar in arteries from both groups, while ACh-induced superoxide anion formation was greater in arteries from orchidectomized than control rats. Orchidectomy increased the vasodilator effect induced by ACh, but did not alter that induced by SNP. SOD, a superoxide anion scavenger, did not modify the SNP-induced relaxation in aortas from control or orchidectomized rats. The membrane-permeable mimetic of SOD, tempol, increased the SNP-induced relaxation more in aortas from orchidectomized than control rats. The effect of endogenous SOD inhibitor, diethyl-dithiocarbamate, reduced the relaxation induced by SNP in segments from both groups of rats. The expression and activity of Cu/Zn-SOD were greater in aortas from orchidectomized than control rats. These data show that endogenous male sex hormone deprivation altered neither eNOS expression nor eNO release, while it increased the expression and activity of Cu/Zn-SOD. However, the predominant vascular effect of orchidectomy is to increase NO metabolism.