G protein signal transduction pathways regulate important aspects of cardiovascular physiology including systemic blood pressure, cardiac growth and cardiac contractile function. Regulator of G protein signaling (RGS) proteins are a large family of GTPase activating proteins (GAPs) that promote the deactivation of heterotrimeric G proteins. RGS4 is a widely-expressed RGS family member that promotes the deactivation of Gi and Gq proteins in vitro. Overexpression of RGS4 in murine heart antagonizes the development of cardiac hypertrophy that occurs in response to pressure overload. Mice were generated that lack RGS4 and these animals appeared normal at birth and were fertile. Rgs4−/− mice had normal cardiac structure and function at 12 weeks of age, and they exhibited a normal left ventricular hypertrophic response to pressure overload by transverse aortic constriction. In response to experimental myocardial infarction by ligation of the left anterior descending coronary artery, rgs4−/− mice exhibited reduced pathological remodeling when compared to wild type mice. Furthermore, rgs4−/− mice exhibited significantly reduced left ventricular infarct size after ex vivo ischemia-reperfusion injury. To determine whether enhanced Gi-mediated signaling was responsible for the cardiac phenotype of rgs4−/− mice, ex vivo cardiac adenosine infusion studies were performed. In these studies, rgs4−/− mice exhibited increased sensitivity to adenosine-mediated depression of cardiac contractile function. These results suggest that RGS4 inhibits ligand-stimulated Gi signaling in myocardium and antagonizes Gi-mediated cardiomyocyte survival pathways.