Prostaglandins have been shown to relax the smooth muscle of the ductus arteriosus in the fetus in utero. This physiologic action has been applied to the management of newborn infants with certain types of congenital malformations. Infants with lesions producing right ventricular outflow obstruction have a compromised pulmonary circulation and require a patent ductus arteriosus for adequate pulmonary blood flow. Infusion of alprostadil (PGE1) dilates the ductus, increases pulmonary blood flow, and thereby improves oxygenation. Likewise, infants with aortic arch interruption or coarctation of the aorta are dependent on an open ductus to maintain lower body perfusion. Alprostadil is of great benefit in this situation as well. The side effects of alprostadil include peripheral vasodilation and hypotension and, most importantly, apnea. Hyperpyrexia and jitteriness may also occur. Side effects occur only in about 20% of infants and usually are easily reversed. The benefits therefore greatly outweigh the risks, but careful monitoring is essential.