Arachidonic acid is metabolized to biologically active substances by three major enzyme systems including cyclooxygenases, lipoxygenases and cytochrome P450s. The third pathway, P450-dependent pathway, includes allylic oxidation, omega-hydroxylation, and epoxidation of arachidonic acid. Of these metabolites, the physiological role of 20-hydroxyeicosatetraenoic acid (20-HETE) produced by CYP4A isoforms has been extensively studied. 20-HETE affects ion transport, constricts blood vessels and participates in tubuloglomerular feed back. Increased production of 20-HETE is a major factor in elevating blood pressure in spontaneously hypertensive rats (SHR). We have found that CYP4A2 level in SHR is much higher than that of normotensive rat. Recently, factors of endothelial origin other than nitric oxide and prostaglandins were reported. Inhibitors of P450-dependent arachidonic acid metabolism greatly reduce the vasodilator effect and this factor is speculated to be an epoxide of arachidonic acid. We have isolated CYP2C23 from rat kidney and have found that it produces arachidonic acid epoxides. We have investigated changes in the CYP2C23 levels in physiological and pathophysiological conditions. Multiple pathways of arachidonic acid metabolism by P450 have been reported and the diverse properties of these metabolites and the wide distribution of the P450 system make them prime candidates for participation in regulatory mechanisms of the circulation and transporting epithelia.