Diabetic patients develop a cardiomyopathy that consists of ventricular hypertrophy and diastolic dysfunction. Although the pathogenesis of this condition is poorly understood, previous studies implicated abnormal G-protein activation. In this work, mice with cardiac overexpression of the transcription factor peroxisome proliferator-activated receptor-α (PPAR-α) were examined as a model of diabetic cardiomyopathy. PPAR-α transgenic mice develop spontaneous cardiac hypertrophy, contractile dysfunction, and "fetal" gene induction. We examined the role of abnormal G-protein activation in the pathogenesis of cardiac dysfunction by crossing PPAR-α mice with transgenic mice with cardiac-specific overexpression of regulator of G-protein signaling subtype 4 (RGS4), a GTPase activating protein for G q and Gi. Generation of compound transgenic mice demonstrated that cardiac RGS4 overexpression ameliorated the cardiomyopathic phenotype that occurred as a result of PPAK-α overexpression without affecting the metabolic abnormalities seen in these hearts. Next, transgenic mice with increased or decreased cardiac Gq signaling were made diabetic by injection with streptozotocin (STZ). RGS4 transgenic mice were resistant to STZ-induced cardiac fetal gene induction. Transgenic mice with cardiac-specific expression of mutant Gαq, G αq-G188S, that is resistant to RGS protein action were sensitised to the development of STZ-induced cardiac fetal gene induction and bradycardia. These results establish that Gq-mediated signaling plays a critical role in the pathogenesis of diabetic cardiomyopathy.