Background Estrogen replacement therapy reduces the risk of coronary heart disease in postmenopausal women, and estrogen treatment modulates endothelium-dependent vasodilation in ovariectomized, atherosclerotic monkeys. Estradiol-17 beta also induces relaxation in isolated rabbit coronary arteries as well as cerebral basilar arteries. The estrogen concentrations required to induce such relaxation are in the pharmacological range (10 sup -6 to 10 sup -5 mol/L).Methods and Results The present study was designed to test whether the sensitivity and specificity of the relaxing response of coronary vascular smooth muscle to exogenous estradiol-17 beta is dependent on the sex hormone status of the animal. In coronary artery rings contracted with PGF2alpha (3 x 10 sup -5 mol/L), estradiol-17 beta caused significant relaxation at a physiological estrogen concentration (10 sup -9 mol/L), in coronary artery rings from oophorectomized, estrogen-treated and acutely estrogen-withdrawn rabbits only. Relaxation induced by estradiol-17 beta at lower concentrations (10 sup -9 to 10 sup -6 mol/L) in these rings was 20+-6%, 42+-8%, 54+-9%, and 75+-8%, respectively, compared with 4+-2%, 12+-5%, 16+-7%, and 25+-12% and 5+-2%, 12+-5%, 18+-8%, and 23+-10% in rings from estrogen-maintained and oophorectomized rabbits, respectively (P<.01). The relaxation in coronary artery rings from estrogen-treated and acutely estrogen-withdrawn rabbits was endothelium and nitric oxide dependent since it was abolished by endothelium removal and the nitric oxide synthase inhibitor Nomega-nitro-l-arginine.Conclusions This study demonstrates that estrogen-induced, endothelium-dependent relaxation of coronary arteries may, in some species, depend on the sex hormone status of the animal. These findings may help to better understand the effects of ovarian steroids in the coronary circulation of females. (Circulation. 1994;90:1964-1968.)