Certain polyunsaturated fatty acids, such as arachidonic acid, are metabolized by oxygenation into a large family of biologically active substances, the prostanoids. These include the prostaglandins, thromboxanes, prostacyclins, leukotrienes and also a number of related compounds. Oxygenation can take place at many different positions of arachidonic acid. A cyclo-oxygenase introduces oxygen at C-11 and converts the resulting peroxy compound into a 9, 11-endoperoxide structure. The cyclic peroxides thus formed, PGG2 and PGH2, are highly potent compounds and are the immediate precursors of the prostaglandins, thromboxanes and prostacyclin. Other enzymes, the lipoxygenases, may instead introduce oxygen at C-5, C-8, C-9, C-12 or C-15: further conversions from, for example, the initially formed 5- or 15-hydroperoxy acids may lead to the leukotrienes. The prostanoids display strong and varied biological activities, and have effects on numerous processes in the body. In some pathological conditions the prostanoids play important roles. For example, certain products of the arachidonic acid cascade are considered to be mediators of the inflammatory response: they are formed during the process, contribute to the symptoms of erythema, vascular leakage, fever, pain and chemotaxis, and inhibition of their biosynthesis can be achieved at different levels by the anti-inflammatory drugs.