erbB2/Her2, a ligandless receptor kinase, has pleiotropic effects on mammalian development and human disease. The absence of erbB2 signaling in cardiac myocytes results in dilated cardiomyopathy in mice, resembling the cardiotoxic effects observed in a subset of breast cancer patients treated with the anti-Her2 antibody herceptin. Emerging evidence suggests that erbB2 is pivotal for integrating signaling networks involving multiple classes of extracellular signals. However, its role in G protein-coupled receptor (GPCR) signaling remains undefined. Because the activation of the MAPK pathway through GPCR signaling is important for cardiac homeostasis, we investigated whether erbB2 is required for GPCR-mediated MAPK signaling in wild-type and heart-specific erbB2 mutant mice. Here we demonstrate that erbB2, but not EGF receptor, is essential for MAPK activation induced by multiple GPCR agonists in cardiac myocytes. erbB2 is immunocomplexed with a GPCR in vivo and is transactivated after ligand treatment in vitro. Coexpression of erbB2 with GPCRs in heterologous cells results in ligand-dependent complex formation and MAPK activation. Furthermore, MAPK activation and cardiac contractility are markedly impaired in heart-specific erbB2 mutants infused with a GPCR agonist. These results reveal an essential mechanism requiring erbB2 as a coreceptor for GPCR signaling in the heart. The obligatory role of erbB2 in GPCR-dependent signaling may also be important in other cellular systems.