XCsx2, a homeobox-containing gene, is expressed in cardiac muscle during Xenopus development, while the XMEF2A gene is expressed in both cardiac and skeletal muscle. Microinjection of either XCsx2 or XMEF2A mRNA into single blastomeres of two-cell stage Xenopus embryos induced precocious expression of the myosin heavy-chain alpha (XMHCα) gene at the neural plate stage (stage 14). Co-injection of both XCsx2 and XMEF2A mRNAs induced still earlier expression at the late gastrula stage (stage 12). These changes were evident in whole embryos but not in animal pole explants from injected embryos. Overexpression of XCsx2 or XMEF2A also caused an enlarged heart and abnormalities of notochord and tail in Xenopus embryos. These findings suggest that both XCsx2 and XMEF2A transcription factors have an important role in regulating the expression of the XMHCα gene and the morphogenesis of heart tissue in Xenopus development.