Chronic stimulation of cardiac β₁‐adrenergic receptors contributes to disease progression and mortality in patients and animal models of heart failure. To search for the mechanism of adrenergic impairment of cardiac function in vivo, we studied transgenic mice with cardiac‐specific overexpression of β₁‐adrenergic receptors. Transgenic mice with cardiac overexpression of β₁‐adrenergic receptors showed progressive left ventricular fibrosis starting at 4 months of age. Left ventricular catheterization revealed a modest enhancement of contractility and relaxation at 2 months of age, followed by progressive dysfunction in both parameters and ultimately cardiac failure. When the effects of endogenous catecholamines were blocked by the β‐receptor antagonist propranolol, maximal rate of contractility (dp/dt_max) and maximal rate of relaxation (dp/dt_min) were significantly blunted in 2‐month‐old β₁‐receptor transgenic mice. Isolated cardiomyocytes from these animals displayed markedly altered calcium transients with significant prolongation of the intracellular calcium transient compared with nontransgenic littermates. We determined the expression of sarcoplasmic reticulum proteins involved in calcium handling by RNase protection assay and by immunoblotting. Although the expression of calsequestrin, triadin, and phospholamban was not altered, we observed a progressive decrease in junctin abundance in β₁‐receptor transgenic mice (P < 0.001 TG vs. WT). Altered expression of the sarcoplasmic reticulum protein junctin may be involved in the pathogenesis of cardiac failure after chronic stimulation of β₁‐adrenergic receptors.