The chicken embryo has two functioning ductus arteriosi (DA) during development. These blood vessels connect the pulmonary arteries to the descending aorta providing a right-to-left shunt of blood away from the nonrespiring lungs and to the systemic circuit and chorioallanotic membrane. The DA consists of two distinct tissue types along its length, a muscular proximal portion and an elastic distal portion. During hatching, the DA must close for proper separation of systemic and pulmonary circulation. We examined the morphological changes of the chicken DA before, during, and after hatching. Occlusion of the proximal DA began during external pipping and was complete at hatching. Anatomical remodeling began as early as external pipping with fragmentation of the internal elastic lamina and smooth muscle actin appearing in the neointimal zone. By day 2 posthatch, the proximal DA lumen was fully occluded by endothelial cells and smooth muscle actin positive cells. In contrast, the distal DA was not fully occluded by day 2 posthatch. Increases in Po 2 of the blood serves as the main stimulus for closure of the mammalian DA. The responsiveness of the chicken proximal DA to oxygen increased during hatching, peaking during external pipping. This peak correlated with an increase in blood gas Po 2 and the initial occlusion of the vessel. The distal portion remained unresponsive to oxygen throughout hatching. In conclusion, the chicken DA begins to close during external pipping when arterial Po 2 increases and vessel tone is most sensitive to oxygen.