Caesarean newborn rats were given subcutaneously enalapril maleate (EM), an angiotensin-converting enzyme inhibitor, (1) immediately or (2) 180 min after delivery. The ductus arteriosus (DA) in these newborn pups was calibrated 30, 60 and 90 min after the EM-treatment. The results were as follows: (1) DA calibers of the pups given 10 or 50 mg/kg EM just at caesarean delivery remained significantly larger than those of the controls thereafter until 90 min. (2) The DA was completely closed 180 min after caesarean delivery. However, with 50 mg/kg EM given at this time, the DA was temporarily re-opened and then constricted. It is concluded that, in newborn rats, EM has the direct inhibitory action on the constriction of the DA, and that it also has the re-opening action on the once-constricted DA.