InDrosophila,thetinmangene is absolutely required for development of the dorsal vessel, the insect equivalent of the heart. In vertebrates, thetinmangene is represented by a small family oftinman-related sequences, some of which are expressed during embryonic heart development. At present however, the precise importance of this gene family for vertebrate heart development is unclear. Using theXenopusembryo, we have employed a dominant inhibitory strategy to interfere with the function of the endogenoustinman-related genes. In these experiments, suppression oftinmangene function can result in the complete elimination of myocardial gene expression and the absence of cell movements associated with embryonic heart development. This inhibition can be rescued by expression of wild-typetinmansequences. These experiments indicate that function oftinmanfamily genes is essential for development of the vertebrate heart.