OBJECTIVE To examine the role of endogenous nitric oxide (NO) and endothelin-1 (ET-1) in the pathogenesis of patent ductus arteriosus of the preterm infants. We hypothesize that the patency of ductus arterious in preterm infants is probably due to inappropriate production of endogenous nitric oxide and the interaction with various inflammatory mediators and prostaglandins, which is different from those of term infants. In addition, the secretion of endothelin is probably decreased. The purpose of this study is to monitor the changes of these substance sequentially, and to evaluate the relationship among endothelin-1, endogenous nitric oxide, and inflammatory mediators in the pathophysiology of patent ductus arteriosus in preterm infants.