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.