Since arachidonic acid is the obligatory substrate for the synthesis of prostaglandins, the regulation of arachidonic acid from phospholipid stores is likely to be pivotal in the release of prostaglandins and the initiation of labour. The hydrolysis of phospholipids to yield arachidonic acid is catalysed by phospholipases of which there are two of particular importance. Phospholipase C mediates the action of certain agonists including oxytocin and acts specifically on phosphatidyl-inositol resulting in the release of inositol phosphates. Phospholipase A2 is activated by a variety of physical and chemical agents (e.g. infection, trauma) that increase calcium concentrations in the cell; it releases arachidonic acid from phosphatidyl-choline and phosphatidyl-ethanolamine in particular. Factors influencing phospholipase activity are important in the mechanism initiating labour whether preterm or term. A specific chorionic protein (gravidin) that inhibits activity of phospholipase A2 during pregnancy but loses its activity at the onset of labour has been identified and characterised.