Arachidonic acid takes part in a number of complicated enzymic processes in which not only the prostaglandins but also the intermediate products of their biosynthesis have important physiological effects. The prostaglandin-synthesizing system can convert a large number of nonnatural substrates, leading to the formation of substituted prostaglandins, some of which have interesting pharmacological properties. The prostaglandin-synthesizing system is found both in the higher animals and in lower animals such as the corals and mussels. So far there have been no indications that this system occurs in the plant kingdom, even in those plants which are able to produce arachidonic acid. It is attractive to assume that this arachidonic acid-prostaglandin system functions as a regulator of certain membrane functions for which there is no parallel need in the plant.