The arachidonic acid epoxygenase metabolite 14,15-epoxyeicosatrienoic acid is further metabolized by rat liver microsomal fractions to regioisomeric diepoxides and epoxyalcohols. Diepoxides result from epoxidation at the 5,6-, 8,9-, or 11,12-olefins. Hydroxylation leading to epoxyalcohols with a cis, trans-conjugated dienol occurs at carbons 5, 8, 9, or 12. Structural assignments were established by chromatographic and mass spectral comparisons with synthetic standards. The reaction requires NADPH and is inhibited by typical cytochrome P-450 inhibitors. Analysis of the time course of product formation during arachidonic acid oxidation by rat liver microsomal fractions indicated that all four regioisomeric epoxyeicosatrienoic acids can be further metabolized by the enzyme system.