The epidermis expresses cyclooxygenases, lipoxygenases, and cytochromes P450, which utilize arachidonic acid to generate a diverse array of lipid mediators affecting epidermal cellular differentiation and functions. Recent studies show that mouse epidermis expresses CYP2B19, a keratinocyte-specific epoxygenase that generates 11,12-and 14,15-epoxyeicosatrienoic (EET) acids from arachidonate. We studied CYP2B19-dependent metabolism in mouse epidermal microsomes, reconstituted in the presence of [1-14 C]arachidonic acid. The majority of the 14 C products formed independently of NADPH, indicative of robust epidermal cyclooxygenase and lipoxygenase activities. We studied two NADPH-dependent products generated in a highly reproducible manner from arachidonate. One of these (product I) coeluted with the CYP2B19 product 14,15-EET on a reversed-phase high-performance liquid chromatography (HPLC) system; there was no evidence for other regioisomeric EET products. Further analyses proved that product I was not an epoxy fatty acid, based on different retention times on a normal-phase HPLC system and failure of product I to undergo hydrolysis in acidic solution. We analyzed purified epidermal 14 C products by liquid chromatography negative electrospray ionization mass spectrometry. Structures of the NADPH-dependent products were confirmed to be 12-oxo-5,8,14-eicosatrienoic acid (I) and 12-hydroxy-5,8,14-eicosatrienoic acid (II). This was the first evidence for a 12-hydroxy-5,8,14-eicosatrienoic acid biosynthetic pathway in mouse epidermis. Epidermal microsomes also generated 12-hydroperoxy, 12-hydroxy, and 12-oxo eicosatetraenoic acids from arachidonate, possible intermediates in the 12-hydroxy-5,8,14-eicosatrienoic acid biosynthetic pathway. These results predict that hydroxyeicosatrienoic acids are synthesized from arachidonate in human epidermis. This would have important implications for human skin diseases given the known pro-and anti-inflammatory activities of stereo-and regioisomeric hydroxyeicosatrienoic acids.Cyclooxygenases, lipoxygenases, and cytochromes P450 (P450 gene products) activate arachidonic acid. Prostanoids, hydroxy and epoxy fatty acids generated by these enzyme systems, mediate signals involved in epidermal homeostasis, skin diseases, and cancers. These lipid mediators are implicated in mechanisms regulating diverse cellular processes including epidermal cell proliferation and differentiation, cutaneous responses to inflammation and environmental cell damage, and cutaneous immune responses (for review, see