To study the formation of the coronary vessels in the developing avian heart, we stained developmentally staged quail hearts with the endothelial specific antibody QH-1. QH-1 reacted with individual cells in the proepicardial organ in Hamburger and Hamilton stage (HH) 17 embryos only after it had contacted the heart. In HH18-26 hearts, individual QH-1+ cells accumulated over the surface of the atria and ventricles. The first endothelial vessels appeared in the dorsal atrioventricular groove in HH23 hearts. CD45+ hematopoietic precursors accumulated on the heart surface, demonstrating the close temporal relationship of hematopoiesis with vasculogenesis during heart development. However, CD45 expression preceded association of these cells with the vasculature, suggesting hematopoietic commitment precedes formation of blood islands in the coronary vasculature. Endothelial tubules first appeared on the dorsal and then the ventral aspects of the heart, coalescing into large sinusoids. These sinusoids remodeled into compact muscularized vessels by HH35. Smooth muscle cell markers were first expressed at HH27 and only in association with developing vasculature. We did not observe markers of smooth muscle differentiation in the proepicardium, but it remains uncertain whether cells in the proepicardium are committed to this cell fate. Our data support a strictly vasculogenic mechanism for the formation of the coronary vessels and blood islands.