Clinical trials revealed that estrogen may result in cardiovascular risk in patients with coronary heart disease, despite earlier studies demonstrating that estrogen provided cardiovascular protection. It is possible that the pre-existing condition of hypertension and the ability of estrogen to activate the renin-angiotensin system could confound its beneficial effects. Our hypothesis is that attenuation of agonist-induced vasoconstrictor responses through activation of NOS by estrogen is impaired by hypertension. We investigated the effects of 17- estradiol (E₂) replacement in normotensive Sprague Dawley (SD) and m(Ren2)27 hypertensive transgenic (TG) rats on contractile responses to three vasoconstrictors: angiotensin II (Ang II), serotonin (5-HT), and phenylephrine (PE) and on the modulatory role of vascular nitric oxide (NO) to these responses. The aorta was isolated from ovariectomized SD and TG rats treated chronically with 5 mg 17-beta estradiol or placebo (P). Isometric tension of aortic rings was measured in organ chambers, and endothelial NO synthase in rat aorta was detected using western blotting. E₂ treatment increased eNOS expression in SD and TG aorta and reduced Ang II and 5HT but not PE-induced contractions in SD and TG rats. Inhibition of NOS with N-nitro-L-arginine methyl ester enhanced Ang II, 5HT and PE-induced contractions in P-treated and Ang II and PE responses in E₂-treated SD and TG rats. Only the responses to 5HT were augmented in hypertensive rats. In conclusion, this study shows that the pre-existing condition of hypertension augmented the vascular responsiveness to 5HT, whereas attenuation of Ang II and 5HT vascular responses by estrogen were not impaired by hypertension. The adrenergic agonist was unresponsive to estrogen treatment. The contribution of NO as a factor contributing to the relative refractoriness of the vascular responses is dependent upon the nature of the vasoconstrictor and/or the presence of estrogen.