A renewed study of the development of the branchial arch system was essential in view of the special morphologic characteristics of the ductus arteriosus, which derives from the pulmonary arch artery or sixth branchial arch artery. In congenital heart disease certain aorto-pulmonary collateral arteries have a marked histological similarity to the ductus arteriosus. To gain a better insight into the development of these vessels, 27 rat embryos, with the number of somites ranging between 19 and 41, were studied. Most embryos were collected after shortterm in vitro-culture, allowing precise staging of age and development. The vascular system of these embryos was injected with Indian ink, to enable easy recognition of even the smallest endothelium-lined vessels. The embryos were serially sectioned (3-5 microns) and reconstructed using a graphic method. The results show that the pulmonary arch artery differs from the other arch arteries in that it is the most cranial vessel of a system of ventral splanchnic arteries, which connects the pulmonary plexus with the dorsal aortae at an early stage. With the exception of the pulmonary arch artery, these connections are transient. The pulmonary arteries develop from the remaining parts of the plexus. It is argued that these connections can persist in the human as aorto-pulmonary collaterals, in certain cases with abnormalities in the pulmonary part of the cardiac outflow tract.