ABSTRACT: A method for the detection and quantification of hydroxyl and epoxy arachidonic acid (AA) metabolites in human plasma was developed using liquid-liquid extraction, phospholipid saponification followed by derivatization of the acid moiety and liquid chromatographic tandem mass spectrometric detection. Derivatization with a pyridinium analog allowed for detection in the positive ion mode, greatly improving sensitivity and the stability of the more labile AA metabolites. The entire method utilizes a 96-well plate format, increasing sample throughput, and was optimized to measure 5-, 8-, 9-, 11-, 12-, 15-, 19-, and 20- hydroxyeicosatetraenoic acid (HETE), 5,6-, 8,9-, 11,12-, and 14,15- dihydroxyeicosatrienoic acid (DHET), and the regio- and cis-/ trans- isomers of 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acid (EET). The method was validated for its applicability over the FA concentration range found in human plasma. Using 100??L aliquots of pooled human plasma, EET levels, particularly 5,6-EET, were observed to be higher than previously reported, with measured concentrations of 23.6?ng/ml for 5,6-EET, 5.6?ng/mL for 5,6-trans-EET, 8.0?ng/mL for 8,9-EET, 1.9?ng/mL for 8,9-trans-EET, 8.8?ng/mL for 11,12-EET, 3.4?ng/mL for 11,12-trans-EET, 10.7?ng/mL for 14,15-EET, and 1.7?ng/mL 14,15-trans- EET. This method is suitable for large population studies to elucidate the complex interactions between the eicosanoids and various disease states and may be used for quantitation of a wide variety of fattyacids beyond eicosanoids from small volumes of human plasma.