Lipid peroxidation occurs in animal tissue when antioxidant protective systems are lacking. Among the protective systems in animals are the biological antioxidant vitamin E and selenium-glutathione peroxidase, a decomposer of peroxides. Lipid peroxidation also occurs when a variety of chemical oxidants are introduced into a animal. The most recently developed technique for measurement of lipid peroxidation in vivo is the analysis of expired air for volatile hydrocarbon products of lipid hydroperoxide decomposition. Pentane is derived from the major fatty acids of the animal body, those of the omega 6-un-saturated fatty acid family, while ethane is derived from omega 3-unsaturated fatty acids. The technique of measuring volatile hydrocarbons has been used by a number of investigators to show increased lipid peroxidation in animals fed diets deficient in antioxidant nutrients and in animals exposed to ozone or injected with iron, halogenated hydrocarbons, or peroxides. Evidence that volatile hydrocarbons are produced during lipid peroxidation, the similarities of results obtained by different investigators who have measured lipid peroxidation by volatile hydrocarbon production, and the relative effects of nutritional and chemical treatments on production of volatile hydrocarbons by laboratory animals or humans are reviewed.