Prostaglandins appear to play an important role as mediators of inflammation since they fulfill the major criteria. They possess powerful proinflammatory properties; they are present in increased concentrations in a wide range of proinflammatory lesions, and anti-inflammatory drugs inhibit their formation. The role of leukotrienes and other hydroxy fatty acid products is far less clear although their participation in cellular events seems probable. Steroid and nonsteroid anti-inflammatory drugs owe their activity at least in part to inhibition of biosynthesis of prostaglandins, although these two drug classes appear to act at different points in the pathways. It becomes increasingly clear that the oxygenation of arachidonic acid leads to the formation of a multiplicity of pharmacologically active fatty acids of which only a few have so far been identified.