Studies in premature animals suggest that 1) prolonged tight constriction of the ductus arteriosus is necessary for permanent anatomic closure and 2) endogenous nitric oxide (NO) and prostaglandins both play a role in ductus patency. We hypothesized that combination therapy with an NO synthase (NOS) inhibitor [N G -monomethyl-L-arginine (L-NMMA)] and indomethacin would produce tighter ductus constriction than indomethacin alone. Therefore, we conducted a phase I and II study of combined treatment with indomethacin and L-NMMA in newborns born at Ͻ28 weeks' gestation who had persistent ductus flow by Doppler after an initial three-dose prophylactic indomethacin course (0.2, 0.1, 0.1 mg/kg/24 h). Twelve infants were treated with the combined treatment protocol [three additional indomethacin doses (0.1 mg/kg/24 h) plus a 72-hour L-NMMA infusion]. Thirty-eight newborns received three additional indomethacin doses (without L-NMMA) and served as a comparison group. Ninety-two percent (11/12) of the combined treatment group had tight ductus constriction with elimination of Doppler flow. In contrast, only 42% (16/38) of the comparison group had a similar degree of constriction. L-NMMA infusions were limited in dose and duration by acute side effects. Doses of 10 -20 mg/kg/h increased serum creatinine and systemic blood pressure. At 5 mg/kg/h, serum creatinine was stable but systemic hypertension still limited L-NMMA dose. We conclude that combined inhibition of NO and prostaglandin synthesis increased the degree of ductus constriction in newborns born at Ͻ28 weeks' gestation. However, the combined administration of L-NMMA and indomethacin was limited by acute side effects in this treatment protocol. Studies in premature animals demonstrate that prolonged, tight constriction of the ductus arteriosus is necessary to produce ischemia of the ductus wall, anatomic remodeling, and permanent closure (1-3). A persistent patent ductus arteriosus (PDA) in premature infants is due in large part to the increased sensitivity of the ductus to endogenous vasodilators like prostaglandins. Prostaglandin synthesis inhibitors, such as indomethacin, are the only drugs currently approved for treatment of PDA. Unfortunately, a three-dose course of indomethacin frequently fails to close the PDA in infants of Ͻ28 weeks gestational age (GA). Previous studies have shown that infants delivered before 28 weeks GA who still have evidence of ductus flow on Doppler examination (performed after a standard three-dose course of indomethacin), are unlikely to eliminate Doppler flow through their ductus even if a prolonged six-dose indomethacin course is administered (2).