The nature and mechanism of formation of the thiobarbituric acid (TBA)-reaction material produced in the autoxidation of polyunsaturated fatty acids (PUFA) or their esters has been studied. On the basis of chemical studies and spectroscopic evidence, it is concluded that the TBA test detects malonaldehyde which arises at least in part from the acid-catalyzed or thermal decomposition or endoperoxides (2,3-dioxanorbornane compounds). These endoperosides have structures related to those of the endoperoxides produced in the biosynthetic sequence leading to prostaglandins. A mechanism is proposed in which these endoperoxides are formed in a free radical cyclization process operating in competition with hydroperoxide formation during the autoxidation of PUFA or their esters containing three or more double bonds. When 20:3 or 20:4 PUFA undergo autoxidation, some of the natural, physiologically active prostaglandins would be produced, although in very low yield, along with many other stereo- and positional isomers. Thus, it is possible that some of the complex symptoms of lipid peroxidation in vivo could be due to nonenzymatically produced prostaglandins or their steroisomers.