To investigate which factors are involved in the differentiation of Bufo arenarum heart myoendocrine cells, we studied the distribution of Atrial Natriuretic Peptide (ANP) immunoreactivity in hearts formed from presumptive cardiac mesoderm explanted at early embryo stages. Explants isolated from different embryonic stages throughout neurulation were cultured in vitro with or without the pharyngeal endoderm, and in other cases transplanted to the caudal region of embryos at similar stages. We utilized immunohistochemical and morphological techniques to assess myoendocrine cardiac cell differentiation. Development of heart beat and positive tropomyosin immunolabeling were considered signs of cardiac tissue differentiation. Our results confirm that explants of cardiac mesoderm cultured with endoderm showed a greater and more complete level of cardiac differentiation than those of mesoderm alone, and this coincides with the staining pattern of tropomyosin. ANP immunostaining revealed that cardiac cells containing ANP were scarce in those cultures without endoderm. On the contrary, in both cultured and grafted explants containing endoderm, ANP immunostaining was intensive and well-distributed in the cardiac tube, and in some cases restricted to one side of the formed heart. We conclude that the endoderm regulates cardiac cell differentiation, and in this way, is involved in the development of the heart myoendocrine system.