The biosynthesis of prostaglandins is a very rapid reaction. Since blood platelets, when disturbed, readily convert e.g. arachidonic acid into prostaglandin E2 and F2 alpha, those compounds cannot be quantitated in body fluids or tissues which contain platelets. When present in the circulation, primary prostaglandins are rapidly inactivated to corresponding 15-keto-13, 14-dihydro metabolites which occur in plasma in about ten times higher concentrations than the primary prostaglandins. The basal plasma levels of these metabolites are lower than 100 picog/ml. Therefore highly sensitive and specific techniques are necessary for safe quantitation of prostaglandins. The rapid inactivation of prostaglandins also makes it necessary to carefully determine which metabolite should be quantitated in a given experimental situation. The complexity of prostaglandin biochemistry and its importance for accurate monitoring of prostaglandin synthesis in different experimental situations will be discussed in this article.