Several lines of evidence implicate E-type PGs in the maintenance of patency of the foetal ductus arteriosus. The PGE mechanism is functional in man, sheep, rat and rabbit, but it is seemingly absent in guinea pig. The role of the PGs in the closure of the vessel at birth is uncertain. Work with different species suggests that as PO2 rises at birth the relaxant effect of PGEs on the ductal muscle decreases. This decreased sensitivity may facilitate the oxygen triggered contraction. The calf ductus is an exception and its closure may be mediated by PGF2alpha. An important fact emerging from this review is that species may differ with regard to the occurrence and possible function of the PGs in the ductus arteriosus. The significance of these differences to the function of the oxygen-sensing mechanism remains a major question for future research.