Recent studies from our laboratory have indicated that vascular smooth muscle cells (VSMC) metabolize arachidonic acid via a P4504A-dependent pathway to 20-hydroxyeicosatetraenoic acid (20-HETE), and that this system serves as a novel signal transduction pathway that plays a central role in the regulation of vascular tone. The major metabolite of arachidonic acid formed in cerebral and renal arteries is 20-HETE. The mRNA and protein for P4504A enzymes, which produce 20-HETE, have been localized in VSMC. 20-HETE is a potent vasoconstrictor, that acts in part by inhibition of the opening of the large conductance, calcium-activated potassium channel, and depolarizes VSMC membrane. A preliminary study also indicated that 20-HETE activates the L-type calcium current in cerebral arterial smooth muscle. Inhibition of the endogenous production of 20-HETE in renal and cerebral arterioles attenuates pressure-dependent myogenic tone in vitro, as well as autoregulation of renal and cerebral blood flow in vivo. There is also evidence that indicates that nitric oxide regulates the formation of 20-HETE by binding and inactivating the P450 heme moiety, thus providing a negative feedback control mechanism for this system. The data outlined suggest that 20-HETE could act as a intracellular second messenger that plays an integral role in the signal transduction processes underlying the development of pressure-dependent myogenic tone.