Oxygen constriction causes functional closure of the ductus arteriosus (DA) at birth. Although DA closure is crucial for postnatal adaptation, patency of the DA is critical for survival of newborns with duct-dependent cardiac malformations. In these cases, DA patency is achieved by i.v. infusion of prostaglandin E1, which, though effective, is often associated with complications. We hypothesized that sildenafil, a specific phosphodiesterase type 5 inhibitor, is an effective DA vasodilator. In isolated DA rings from term (d 30) fetal rabbits, sildenafil (10 Ϫ6 -10 Ϫ4 M) and diethylamine NONOate (10 Ϫ7 -10 Ϫ5 M) induced dosedependent relaxation of oxygen-constricted DA (Ϫ52 Ϯ 4% and Ϫ51 Ϯ 6%, respectively) that was inhibited by the soluble guanylyl-cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (5 ϫ 10 Ϫ5 M). Sildenafil increased cyclic GMP levels. Iberiotoxin (200 nM), an inhibitor of calcium-sensitive potassium channels, decreased the vasodilatory effect of sildenafil and diethylamine NONOate (Ϫ30 Ϯ 2% and Ϫ27 Ϯ 4%, respectively). Oxygen inhibition of whole-cell K ϩ current and membrane depolarization were partially restored by sildenafil, and this was inhibited by iberiotoxin. Immunohistochemistry and immunoblotting confirmed the presence of phosphodiesterase type 5 and calcium-sensitive potassium channels in the DA smooth muscle cells. This is the first study to demonstrate that sildenafil dilates the DA by increasing soluble guanylyl-cyclase-derived cGMP levels and thereby activating calcium-sensitive potassium channels, causing membrane hyperpolarization. Sildenafil, already approved for human usage, might be an alternative or a useful adjunct to prostaglandin E1 as a bridge to cardiac surgery. The DA is a vital fetal structure that connects the pulmonary artery to the descending aorta. During fetal life, the DA shunts over half of the blood flow away from the lung into the umbilico-placental circulation, where gas exchange takes place (1). At birth, closure of the DA, essential for postnatal adaptation, is initiated by an increase in O 2 . In full-term newborns, the DA closes 24 -48 h after delivery (1). However, in newborns with duct-dependent cyanotic cardiac malformations (e.g. transposition of the great vessels) or duct-dependent aortic obstructions (including hypoplastic left heart syndrome and coarctation of the aorta), patency of the DA is critical for survival until surgery. The paramount importance of PG in the regulation of ductal tone is well established [reviewed in (2)]. This has resulted in successful pharmacological manipulation of the DA (3). To date, PGE1 is the only available therapy to maintain DA patency (4). Although effective, numerous side effects are associated with PGE1 infusions (respiratory depression, fever, lethargy, irritability, myoclonic jerks, flushing, edema, pyloric stenosis, hyperostosis, necrotizing enterocolitis,