It has been established that coronary vessels develop through self-assembly of mesenchymal vascular progenitors in the subepicardium. Mesenchymal precursors of vascular smooth muscle cells and fibroblasts are known to originate from an epithelial-to-mesenchymal transformation of the epicardial mesothelium, but the origin of the coronary endothelium is still obscure. We herein report that at least part of the population of the precursors of the coronary endothelium are epicardially-derived cells (EPDCs). We have performed an EPDC lineage study through retroviral and fluorescent labelling of the proepicardial and epicardial mesothelium of avian embryos. In all the experiments onlythe surface mesothelium was labelled after 3 h of reincubation. However, endothelial cells from subepicardial vessels were labelled after 24-48 h and endothelial cells of intramyocardial vessels were also labelled after 48-96 h of reincubation. In addition, the development of the coronary vessels was studied in quail-chick chimeras, obtaining results which also support a mesothelial origin for endothelial and smooth muscle cells. Finally, quail proepicardial explants cultured on Matrigel showed colocalization of cytokeratin and QH1 (mesothelial and endothelial markers, respectively) after 24 h. These results, taken together, suggest that EPDC show similar competence to that displayed by bipotential vascular progenitor cells [Yamashita et al., Nature 408: 92-96 (2000)] which are able to differentiate into endothelium or smooth muscle depending on their exposure to VEGF or PDGF-BB. It is conceivable that the earliest EPDC differentiate into endothelial cells in response to myocardially-secreted VEGF, while further EPDC would be recruited by the nascent capillaries via PDGFR-beta signalling, giving rise to mural cells.