ABSTRACT W The dilating effect of prostaglandin (PG) El and E2 on the onceconstricted ductus arteriosus (DA) was examined in newborn rats. The animals were given subcutaneously PGs 3hr after caesarean delivery. The ratio of the DA to the pulmonary artery (PA) was determined 15, 30, 60, 90 and 180 min after injection. Both PGEI and PGE2 dilated the DA for over 90 min. The maximal effect appeared 15 or 30 min after injection. The DA/PA ratio was significantly higher in PGEZ than in PGEl. In addition, another series of newborn rats were given subcutaneously PGE2 6 hr after cacsarean delivery to examine the age-related changes in the DA-dilating effect. The DA/PA ratio was significantly decreased in 6-hr-old pups than in 3-hr-old pups. These findings indicate that both PGE1 and PGE2 have a dilating effect on the once~ constricted DA and that the effect decreases with time.