Hydrocortisone, prednisolone, and betamethasone were injected into a rat that was 21 days pregnant, and the fetal ductus arteriosus was studied using the rapid whole-body freezing technique. All these glucocorticoids constricted the fetal ductus arteriosus in a dose-dependent relationship. Fetal ductal constriction was maximum 1 to 4 hr after injection and was significantly more prominent in premature fetuses than in mature ones.SpeculationGlucocorticoids may be useful in constricting the patent ductus arteriosus in the premature babies in place of indomethacin, which is contraindicated in the presence of renal failure or jaundice. Administration of glucocorticoids to the mother may cause fetal ductal constriction and fetal distress.