F2-isoprostanes are a complex mixture of isomers formed in four regioisomeric family types by free radical-initiated oxidation of arachidonic acid present in membrane phospholipids. F2-isoprostanes isolated from the livers of rats treated with carbon tetrachloride were separated by initial reverse phase HPLC and detected using electrospray ionization mass spectrometry with the characteristic loss of 44 u (C2H4O) from the common 1,3-diol cyclopropane ring found in these eicosanoids. Collision induced decomposition of the carboxylate anions from the separated F2-isoprostanes formed abundant ions characteristic for regioisomers of Type I (m/z 115), Type III (m/z 127), and Type IV (m/z 193), which made possible characterization of these three family subtypes by LC/MS/MS. Capillary GC/MS was employed to further identify the F2-isoprostane regioisomers using electron ionization mass spectrometry and to obtain characteristic mass spectra of the pentafluorobenzyl ester trimethylsilyl ether derivatives. Quantitation of the F2-isoprostanes separated by both reverse-phase HPLC and capillary GC/MS was carried out using negative ion chemical ionization mass spectrometry. The most abundant isomers identified were Type I and IV regioisomers constituting 33 and 25% of the total products, respectively. As expected, the Type II and III regioisomer products were of less abundance. Over 45 F2-isoprostanes could be separated in this complex mixture, suggesting random production of each regioisomeric subtype in this in vivo model.