W e examined the role of p38α MAPK in mediating cardiomyopathy in mice overexpressing β₁-adrenergic receptor (β₁-AR) or β₂-AR by mating them with dominant-negative p38α (DNp38α) MAPK mice. Both β₁-AR and β₂-AR Tg mice had enhanced LV ejection fraction (LVEF) as young adults and developed similar cardiomyopathy at 11–15 months, characterized by reduced LVEF, myocyte hypertrophy, fibrosis, and apoptosis. We inhibited p38α MAPK by mating β₁-AR Tg and β₂-AR Tg mice with DNp38α MAPK mice, which rescued the depressed LVEF and reduced apoptosis and fibrosis in bigenic β₂-AR × DNp38α MAPK mice, but not bigenic β₁-AR × DNp38α MAPK mice, and failed to reduce myocyte hypertrophy in either group. G_sα was increased in both β₁-AR Tg and β₂-AR Tg mice and was still present in bigenic β₁-AR × DNp38α MAPK mice, but not bigenic β₂-AR × DNp38α MAPK mice. This suggests that p38α MAPK is one critical downstream signal for the development of cardiomyopathy following chronic β₂-AR stimulation, but other kinases may be more important in ameliorating the adverse effects of chronic β₁-AR stimulation.