The objective of this study was to determine how the coronary artery stems develop in the chick embryo. The hearts of 51 inkinjected and cleared chick embryos, aged embryonic days 6, 6.5, 7, 7.5, 9, and 10, were dissected, examined, and selectively photographed. Two representative hearts from each group were paraffin embedded, serially sectioned at 10 ¨m, and examined for aortic endothelial budding. We found that the proximal coronary artery did not appear to grow outward from the aorta as commonly described in the literature. It appeared to originate from a capillary ring which encircled the aortic and pulmonary outflow tracts. On embryonic day 7.5, one to three channels arising from this ring penetrated each aortic sinus, in an area of darker textured endothelium. Histologically and grossly, multiple channels were still apparent on day 9, particularly in the left coronary artery. One of these channels always became dominant to form the stem. Each stem, which varied in length from embryo to embryo, always ended in a plexus of sinusoidal endothelial tubes. By day 10, the coronary artery stems were longer, with many major branches. Histologically, evidence of multiple channels still was visible. It is significant that channels from the bulbar vascular ring penetrated the aorta at very specific points in the aortic sinuses and did not penetrate the pulmonary trunk or other aortic sites. We believe this fact indicates that the penetration of the aortic sinuses by channels from the bulbar vascular ring represents a controlled invasion of the aorta.