Myocardin is a muscle lineage-restricted transcriptional coactivator that has been shown to transduce extracellular signals to the nucleus required for SMC differentiation. We now report the discovery of a myocardin/BMP10 (where BMP10 indicates bone morphogenetic protein 10) signaling pathway required for cardiac growth, chamber maturation, and embryonic survival. Myocardin-null (Myocd) embryos and embryos harboring a cardiomyocyte-restricted mutation in the Myocd gene exhibited myocardial hypoplasia, defective atrial and ventricular chamber maturation, heart failure, and embryonic lethality. Cardiac hypoplasia was caused by decreased cardiomyocyte proliferation accompanied by a dramatic increase in programmed cell death. Defective chamber maturation and the block in cardiomyocyte proliferation were caused in part by a block in BMP10 signaling. Myocardin transactivated the Bmp10 gene via binding of a serum response factor-myocardin protein complex to a nonconsensus CArG element in the Bmp10 promoter. Expression of p57kip2, a BMP10-regulated cyclin-dependent kinase inhibitor, was induced in Myocd-/- hearts, while BMP10-activated cardiogenic transcription factors, including NKX2.5 and MEF2c, were repressed. Remarkably, when embryonic Myocd-/- hearts were cultured ex vivo in BMP10-conditioned medium, the defects in cardiomyocyte proliferation and p57kip2 expression were rescued. Taken together, these data identify a heretofore undescribed myocardin/BMP10 signaling pathway that regulates cardiomyocyte proliferation and apoptosis in the embryonic heart.