This review has necessarily been incomplete. We have not considered the potentially important interplay between PGs and male and female reproductive functions or the well-documented relationships between PGs and inflammation. We have examined the evidence for the influence of PGs on the pulmonary and peripheral vascular circulations and the interaction between PGs and the kidney and the autonomic nervous system. Emphasis has been placed on the role of PGs in the control of the circulation of the ductus arteriosus, and our recent experiences with indomethacin in sick, preterm infants with large left-to-right ductal shunting have been outlined. Existing information has been reviewed concerning PGs and the fetal-placental and fetal-maternal units. It should be clear that a host of proposed functions exists for PGs; many remain tentative in light of the conflicting and often bewildering results of animal experimentation.