This work was conducted to determine whether the angiotensin-converting enzyme inhibitors (ACEIs) (enalapril and captopril) administered to mother rats prenatally can potentiate a re-opening of the neonatal ductus arteriosus (DA) induced by prostaglandin E2 (PGE2) after postnatal closure. A subcutaneous injection of PGE2 (4 micrograms) was administered to newborn rats 3 hr after a Cesarean delivery from females which had been orally given 0.1, 1 or 10 mg/kg/day of enalapril or 15 or 150 mg/kg/day of captopril from day 14 to day 20 of gestation. The ratio of the DA to the pulmonary artery (PA) was determined at intervals after the injection. The DA/PA ratio was significantly higher in the newborn rats of mothers who were transplacentally administered these agents compared to the controls, except at the low dose (0.1 mg/kg) group of enalapril. We found that the level in the neonatal lungs of 15-hydroxy prostaglandin dehydrogenase, a key enzyme that catalyzes PGE2 to convert it to its inactive metabolite 15-keto-PGE2, was not affected after maternal treatment with enalapril or captopril. These results indicate that the increased ductal responsiveness to PGE2 in newborn rats was a common response after maternal ACEI treatment, but the catabolism of PGE2 in the lungs did not contribute to this response.