A cardiac homeobox-containing gene Csx/Nkx2-5, which is essential for cardiac development, is abundantly expressed in the adult heart as well as in the heart primordia. Targeted disruption of this gene results in embryonic lethality due to abnormal heart morphogenesis. To elucidate the role of Csx/Nbx2-5 in the adult heart, we generated transgenic mice which overexpress human Csx/Nkx2-5. The transgene was expressed abundantly in the heart and the skeletal muscle. mBNA levels of several cardiac genes including nartriuretic peptides, CARP, MLC2 upsilon, and endogenous Csx/Nkx2-5 were increased in the ventricle of the transgenic mice. Electron microscopic analysis revealed that the ventricular myocardium of the transgenic mice had many secretory granules, which disappeared after administration of vasopressin. These results suggest that Csx/Nkx2-5 regulates many cardiac genes and induces formation of secretory granulesinthe adult ventricle.