We recently reported the discovery that a series of novel prostaglandin (PG)F2-like compounds (F2-isoprostanes) are produced in vivo independent of the cyclooxygenase as products of free radical-catalyzed lipid peroxidation. F2-isoprostanes are initially formed in situ from arachidonic acid esterified to phospholipids and then released preformed. We have now investigated whether PGD2/E2-like isoprostanes are also produced by rearrangement of the PGG2-like intermediates involved in isoprostane formation. Using a variety of approaches utilizing mass spectrometry, compelling evidence was obtained for the presence of D2/E2-isoprostane-containing phosphospholipids in the liver (85 +/- 33 ng/g of liver) and free D2/E2-isoprostanes in the circulation (215 +/- 90 pg/ml) of rats treated with CCl4 to induce lipid peroxidation. In untreated rats, levels of D2/E2-isoprostanes esterified in liver phospholipids were much lower (0.90 +/- 0.10 ng/g), and free compounds could not be detected in the circulation (< 5 pg/ml). Interestingly, one of the E2-isoprostanes that would be expected to be formed in abundance, 8-epi-PGE2, was found to be a potent renal vasoconstrictor, and these effects could be abrogated by SQ29548, a thromboxane receptor antagonist. Further understanding of the biological consequences of the formation of these novel compounds and factors that influence their formation may provide valuable insights into the pathophysiology of oxidant injury.