Drosophila tinman is an NK-class homeobox gene required for formation of the dorsal vessel, the insect equivalent of the vertebrate heart. Vertebrate sequences related to tinman, such as mouse Nkx-2.5, chicken cNkx-2.5, Xenopus XNkx-2.5 and XNkx-2.3 are expressed in cardiac precursors and in tissues involved in induction of cardiac mesoderm. Mice which lack a functional Nkx-2.5 gene die due to cardiac defects. To determine the role of tinman-related sequences in heart development, we have overexpressed both XNkx-2.3 and XNkx-2.5 in Xenopus laevis embryos. The resulting embryos are morphologically normal except that they have enlarged hearts. The enlarged heart phenotype is due to a thickening of the myocardium caused by an increase in the overall number of myocardial cells (hyperplasia). Neither ectopic nor precocious expression of cardiac differentiation markers is detectable in overexpressing embryos. These results suggest that both XNkx-2.3 and XNkx-2.5 are functional homologues of tinman, responsible for maintenance of the heart field.