To test the hypothesis that estrogen confers cardioprotection by suppressing the expression of ␤-adrenoceptor (␤-AR), we first correlated the infarct size in response to ischemic insult and ␤-AR stimulation with the expression of ␤ 1 -AR in sham, ovariectomized (Ovx) and estrogen replaced (Ovx ϩ E 2 ) rats. When ␤-AR is being activated during ischemia, the infarct size was significantly greater in Ovx than in the sham and Ovx ϩ E 2 rats. There is a negative correlation between the infarct size and the expression level of ␤ 1 -AR as revealed by Western blotting and supported by binding analysis. Incubation of ventricular myocytes from Ovx rats with estrogen at 10 Ϫ9