The β-adrenergic receptor kinase 1 (βARK1) is a member of the G protein-coupled receptor kinase (GRK) family that mediates the agonist-dependent phosphorylation and desensitization of G protein-coupled receptors. We have cloned and disrupted the βARK1 gene in mice by homologous recombination. No homozygote βARK1 ^−/− embryos survive beyond gestational day 15.5. Prior to gestational day 15.5, βARK1 ^−/− embryos display pronounced hypoplasia of the ventricular myocardium essentially identical to the “thin myocardium syndrome” observed upon gene inactivation of several transcription factors (RXRα, N- myc , TEF-1, WT-1). Lethality in βARK1 ^−/− embryos is likely due to heart failure as they exhibit a >70% decrease in cardiac ejection fraction determined by direct in utero intravital microscopy. These results along with the virtual absence of endogenous GRK activity in βARK1 ^−/− embryos demonstrate that βARK1 appears to be the predominant GRK in early embryogenesis and that it plays a fundamental role in cardiac development.