Arachidonic acid (AA) is a widely distributed polyunsaturated fatty acid once liberated from phospholipids by phospholipases. AA can undergo either monooxygenation or epoxidation by enzymes in the cytochrome P450 (CYP450) family. CYP-AA metabolites include 19- and 20-hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatrienoic acids (EETs) and diHETEs. These metabolites have different biological functions based on sites of production- the endothelium of vessels in many organs, the lung, the tubular and corneal epithelium, the liver. The likely targets are either enzymes (Na+-K+-ATPase) or ion channels (calcium activated potassium channels). The involvement of conventional receptor physiology in the tissue responses to CYP-AA products remains an open question. Several isoforms of CYP families that metabolize AA have been identified and cloned.