The effects of indomethacin and a beta-sympathomimetic agent on the fetal ductus arteriosus during treatment of premature labor: a randomized double-blind study. To study the effect of maternal indomethacin or nylidrine hydrochloride treatment on the fetus ductus arteriosus and the pulmonary artery, 27 women with threatened preterm labor between 24 and 34 weeks' gestation were studied by echocardiography. Fourteen women were treated with indomethacin and 13 with nylidrin. Both systolic and diastolic velocities in the ductus increased after administration of indomethacin indicating constriction in nine fetuses, and exceeded the corresponding velocities in the fetuses of the nylidrin group (p = 0.001). However, there were no changes in pulmonary artery flow velocities (p greater than 0.5). In the indomethacin group, there was a significant linear positive relationship between the gestational age and the change in ductal flow velocity. Three of the nine patients with ductal constriction also had tricuspid regurgitation. These findings indicate that indomethacin, not nylidrin, causes transient constriction of the ductus arteriosus and the constrictive response increases with the gestational age. We recommend echocardiographic surveillance of fetal hemodynamics when prostaglandin synthesis inhibitors are used in the treatment of spontaneous preterm labor. 