In the development of a functional myocardium and formation of the coronary vasculature, epicardium-derived cells play an essential role. The proepicardial organ contributes to the developing coronary system by delivering mural cells to the endothelium-lined vessels. In search of genes that regulate the behavior of (pro)epicardial cells, the Ets-1 and Ets-2 transcription factors stand out as strong candidates. In the present study, the hypothesis that Ets transcription factors have a role in proper coronary and myocardial development was tested via antisense technology, by targeting Ets-1 and Ets-2 mRNAs to downregulate protein expression in chicken embryos. The results suggest that hereby the development of the coronary system is hampered, primarily by defects in the process of epithelial-mesenchymal transformation of the mesothelia of the primary and secondary heart fields. This was indicated by a lack of periarterial and epicardial mesenchyme, of peripheral coronary smooth muscle cells, and changes in myocardial morphology. A defect in myocardial perfusion caused by the absence of one or both coronary ostia seems to be “solved” by the development of numerous small fistulae connecting the ventricular lumen with the subepicardially located coronary vessels. The presence of coronary vascular aberrations in the antisense-Ets phenotype enabled us for the first time to study abnormal coronary development in a model that is not lethal to the embryo.