Survival of newborn placental mammals depends on closure of the ductus arteriosus (DA), an arterial connection in the fetus which directs blood away from the pulmonary circulation and towards the placenta where oxygenation occurs(1). Here we show that morphological changes resulting in closure of the DA in mice are virtually identical to those observed in larger mammals, including humans(2), and that maintenance of the DA in the open, or patent, stare in fetal mice is dependent on prostaglandin synthesis, This requirement is absent in mice lacking the prostaglandin E-2 EP4 receptor (EP4(-/-) mice). In EP4(-/-) mice of the 129 strain, remodelling of the DA fails to occur after birth, resulting in a left-to-right shunt of blood and subseqently in death. This suggests that the neonatal drop in prostaglandin E-2 (refs 3-7) that triggers ductal closure is sensed through the EP4 receptor, In contrast, 5% of EP4(-/-) mice of mixed genetic background survive, and selective breeding of these mice leads to a 21% survival rate, suggesting that alleles at other loci can provide an alternative mechanism for ductal closure.