Previous studies demonstrated that prostaglandins are local or tissue hormones which can be released from blood vessel walls. In the present study, we investigated the capacity of bovine ductus arteriosus to synthetize prostaglandins in vitro. After incubation of slices of ductus arteriosus in Krebs' solution with (1-14C) arachidonic acid for 3 hours, more than 40% of the radiolabeled material recovered from the incubating medium were metabolites of arachidonic acid. The major product was indistinguishable from 6 keto-PGF1alpha as determined by its chromatographic motility and resistance to alkaline conversion to PGB. The PGI2 synthetic capacity of the ductus arteriosus, as revealed by the predominance of its major metabolite 6 keto-PGF1alpha, suggests that this metabolic pathway of arachidonic acid may contribute to the hemodynamic changes occurring during fetal life and at birth.