The development of cardiac hypertrophy in response to increased hemodynamic load and neurohormonal stress is initially a compensatory response that may eventually lead to ventricular dilation and heart failure. Regulator of G protein signaling 5 (Rgs5) is a negative regulator of G protein-mediated signaling by inactivating Gα(q) and Gα(i), which mediate actions of most known vasoconstrictors. Previous studies have demonstrated that Rgs5 expresses among various cell types within mature heart and showed high levels of Rgs5 mRNA in monkey and human heart tissue by Northern blot analysis. However, the critical role of Rgs5 on cardiac remodeling remains unclear. To specifically determine the role of Rgs5 in pathological cardiac remodeling, we used transgenic mice with cardiac-specific overexpression of human Rgs5 gene and Rgs5 mice. Our results demonstrated that the transgenic mice were resistant to cardiac hypertrophy and fibrosis through inhibition of MEK-ERK1/2 signaling, whereas the Rgs5 mice displayed the opposite phenotype in response to pressure overload. These studies indicate that Rgs5 protein is a crucial component of the signaling pathway involved in cardiac remodeling and heart failure.