Background: Long-term beta-adrenergic receptor (beta-AR) blockade reduces mortality in patients with heart failure. Chronic sympathetic hyperactivity in heart failure causes sustained beta-AR activation, and this can deplete Ca(2+) in endoplasmic reticulum (ER) leading to ER stress and subsequent apoptosis. We tested the effect of beta-AR blockers on ER stress pathway in experimental model of heart failure. Methods and Results: ER chaperones were markedly increased in failing hearts of patients with end-stage heart failure. In Sprague-Dawley rats, cardiac hypertrophy and heart failure was induced by abdominal aortic constriction or isoproterenol subcutaneous injection. Oral beta-AR blockers treatment was performed in therapy groups. Cardiac remodeling and left ventricular function were analyzed in rats failing hearts. After 4 or 8 weeks of banding, rats developed cardiac hypertrophy and failure. Cardiac expression of ER chaperones was significantly increased. Similar to the findings above, sustained isoproterenol infusion for 2 weeks induced cardiac hypertrophy and failure with increased ER chaperones and apoptosis in hearts. beta-AR blockers treatment markedly attenuated these pathological changes and reduced ER stress and apoptosis in failing hearts. On the other hand, beta-AR agonist isoproterenol induced ER stress and apoptosis in cultured cardiomyocytes. beta-AR blockers largely prevented ER stress and protected myocytes against apoptosis. And beta-AR blockade significantly suppressed the overactivation of CaMKII in isoproterenol-stimulated cardiomyocytes and failing hearts in rats. Conclusions: Our results demonstrated that ER stress occurred in failing hearts and this could be reversed by beta-AR blockade. Alleviation of ER stress may be an important mechanism underlying the therapeutic effect of beta-AR blockers on heart failure.