Prostaglandins play a key role in human labor in that prostaglandins can cause abortion or labor and prostaglandin antagonists can inhibit preterm labor. However, the factors determining prostaglandin synthesis and release in human pregnancy are uncertain. An increase in the synthesis of prostaglandins at the start of labor could result from either increased release of free arachidonic acid or removal of constraint on prostaglandin H synthase activity, or both. The points in the biosynthetic pathway of prostaglandins at which control could be exerted physiologically are examined, with emphasis on the phospholipases.