The purpose of the present study was to determine whether estrogen replacement in ovariectomized rats could modulate arterial diameter responses to β-adrenoceptor activation. Under relaxed conditions (0.1 mM papaverine) there were no differences in the lumen diameter of isolated, pressurized (50 mm Hg) mesenteric arteries from nontreated (191.7 ± 13.8 µm; n = 19) versus those from estrogen-treated (190.1 ± 11 µm; n = 14) ovariectomized Sprague-Dawley rats. In arteries precontracted with noradrenaline (0.3-1 µM), isoprenaline (0.01–10 µM)-induced relaxation was significantly increased in arteries from ovariectomized estrogen-treated rats (52.4 ± 2% of the maximal relaxation induced by 0.1 mM papaverine, vs. 33.3 ± 6.5%; p < 0.01). The half-maximal concentration value was 0.04 ± 0.05 µ M in estrogen-treated rats and 0.4 ± 0.1 µM in nontreated rats (p < 0.01). This response was inhibited by propranolol (1 µM) in both groups to a comparable extent (61.5%), and was unaffected by endothelial removal. Forskolin (0.01–10 µM) induced similar concentration-dependent vasodilation in arteries of both groups of rats with no differences in sensitivity or maximal response. These results suggest that isoprenaline acts through β-adrenoceptors present on vascular smooth muscle and that estrogen replacement enhances the relaxant responses induced by β-adrenoceptor activation by an endothelium-independent mechanism.