Fetal breathing occurs sporadically and is inhibited during periods of hypoxemia, when blood, returning from the placenta, is mainly taking the shortcut through the ductus venosus. The hypothesis tested is that this inhibition might be caused by an expansion of the ductus venosus. Such expansion is pronounced during fetal life but ceases to occur after birth. Regular breathing of newborn lambs was recorded, and it was noted how the breathing was affected when blood, with the aid of a roller pump, was infused from the umbilical arteries into the umbilical veins. Nine lambs were examined, and for a maximal period of 2 min blood was infused into the umbilical veins at a rate of 50–150 mL/min. During 12 infusions, breathing temporarily came to a complete stop; in 30 cases, respiration was only partially inhibited; and in five cases, it was not affected. It is concluded that a very clear breathing inhibition may be obtained with an infusion of blood into the umbilical vein. It is speculated that expansion of the ductus venosus may trigger the inhibition and that the reason the effect varies may have to do with the fact that blood entering the body through the umbilical veins may predominantly take one of two routes: the ductus venosus or the hepatic vessels.