Effects of various prostaglandin types on strips of lamb ductus arteriosus were investigated under anaerobic and aerobic conditions. Prostaglandins E1 and E2 relaxed markedly the anoxic ductus over a dose range from 10−9 to 10−5 M whereas they had little or no effect on the tissue after exposure to oxygen. This is in contrast to papaverine which relaxed the ductus to a similar degree before and after exposure to oxygen. An additional finding was that prostaglandins E1 and E2 are less active on the anoxic tissue depolarized by excess potassium. The greater effectiveness of prostaglandins on the anoxic ductus suggests a role for these compounds in the regulation of the vessel tone during fetal life.