Background—Elevated circulating norepinephrine (NE) has been implicated in causing the profound β-adrenergic receptor (βAR) downregulation and receptor uncoupling that are characteristic of end-stage human dilated cardiomyopathy, a process mediated in part by increased levels of β-adrenergic receptor kinase (βARK1). To explore whether chronic sustained NE stimulation is a primary stimulus that promotes deterioration in cardiac signaling, we characterized a gene-targeted mouse in which activation of the sympathetic nervous system cannot lead to an elevation in plasma NE and epinephrine.Methods and Results—Gene-targeted mice that lack dopamine β-hydroxylase (dbh−/−), the enzyme needed to convert dopamine to NE, were created by homologous recombination. In vivo contractile response to the β1AR agonist dobutamine, measured by a high-fidelity left ventricular micromanometer, was enhanced in mice lacking thedbhgene. In unloaded adult myocytes isolated fromdbh−/−mice, basal contractility was significantly increased compared with control cells. Furthermore, the increase in βAR responsiveness and enhanced cellular contractility were associated with a significant reduction in activity and protein level of βARK1 and increased high-affinity agonist binding without changes in βAR density or G-protein levels.Conclusions—Mice that lack the ability to generate NE or epinephrine show increased contractility associated primarily with a decrease in the level of βARK1 protein and kinase activity. This animal model will be valuable in testing whether NE is required for the pathogenesis of heart failure through mating strategies that cross thedbh−/−mouse into genetically engineered models of heart failure.