Linoleic and alpha-linolenic acids are essential unsaturated fatty acids which were referred to as vitamin F much earlier. Their biological significance is to be converted to eicosapolyenoic acids including arachidonic acid which are the precursors of bioactive eicosanoids such as prostaglandins, thromboxanes and leukotrienes. Arachidonic acid is released from phospholipids by the catalysis of phospholipase A2, and then subjected to oxygenation by 5-lipoxygenase for leukotriene production and that by cyclooxygenase for prostaglandin and thromboxane formation. There are two isozymes of cyclooxygenase (COX-1 and COX-2). COX-2 is a product of an immediate early gene and an inducible enzyme, which is considered to play an important role in inflammation. COX-1 is a house-keeping enzyme found in platelets, kidney, stomach and many other tissues. Nonsteroidal anti inflammatory drugs currently used inhibit both COX-1 and COX-2 more or less, and causes gastric erosion and ulceration by inhibiting local production of prostaglandin E2. In view of such important roles of COX-2 in inflammation, inhibitors specific for COX-2 have been developed as anti-inflammatory drugs without stomach injury.