Transgenic mice were created with cardiac-specific overexpression of the β₂-adrenergic receptor. This resulted in increased basal myocardial adenylyl cyclase activity, enhanced atrial contractility, and increased left ventricular function in vivo; these parameters at baseline in the transgenic animals were equal to those observed in control animals maximally stimulated with isoproterenol. These results illustrate a useful approach for studying the effect of gene expression on cardiac contractility. Because chronic heart failure in humans is accompanied by a reduction in the number of myocardial β-adrenergic receptors and in inotropic responsiveness, these results suggest a potential gene therapy approach to this disease state.