AT BIRTH the patent ductus arteriosus has fulfilled its function and should obliterate itself. Various reasons have been presented for its failure to close, but none have been convincing. Why so few ducti remain permanently open seems a more reasonable question. Why should a large endothelium-lined tube between vessels carrying different intraluminal pressures close? Until further evidence is gained all we can do is lamely ascribe the tendency toward spontaneous closure to the fundamental characteristic of the tissue. Many ducti do not close promptly at birth. Christie1 has pointed out that 44 per cent are still open at 1 month of age, 12 per cent at 2 months of age, 2 per cent at 8 months and approximately 1 per cent at 1 year of age. A few ducti close spontaneously between the first and second years of life. It is generally assumed that a ductus which is open