In liver tissue, brain, lungs and skeletal muscles of rats, subjected to overheating, content of conjugated diens and lipid peroxides was distinctly increased. The decrease in the period of malon dialdehyde induction and in the antioxidative activity of lipids correlated with the alteration in content of peroxidation products. The overheating of animals was accompanied by a decrease in unsaturation of lipids in liver tissue, brain and skeletal muscles due to a decrease in amounts of arachidonic acid and to an increase in content of palmitinic and stearinic acids. In lipids of lungs the unsaturation was increased as a result of increase in content of linoleic acid. Administration of alpha-tocopherol and ionol, 10 min before the overheating and in hypothermia, exhibited a protective effect, thioTEPA caused an opposite action. Administration of alpha-tocopherol and ionol 24 hrs before the thermic treatment decreased the survival period of the animals which were subjected to the overheating.