In several species prostaglandin E2 (PGE2) appears to be the most important prostanoid to regulate patency of the ductus arteriosus. We measured the arterial plasma concentration of PGE2 in 29 premature infants (17 with respiratory distress syndrome alone and 12 with respiratory distress syndrome and patent ductus arteriosus) to determine if there was an association between circulating PGE2 and patent ductus arteriosus. The PGE2 concentrations during the first two days after birth were higher than PGE2 concentrations after the third postnatal day (17.1 +/- 3.1 pg/ml, mean +/- SEM, n = 23, vs 6.12 +/- 0.63 pg/ml, n = 19, P < .005). By the third postnatal day, arterial concentrations were similar to adult venous values. There was no significant difference between the plasma PGE2 concentrations in the 12 infants with patent ductus arteriosus and the concentrations in the 17 infants who had no signs of patent ductus arteriosus. These findings do not support the hypothesis that an increased concentration of circulating PGE2 is the primary cause for the persistent patency of the ductus arteriosus in preterm infants with respiratory distress syndrome.