Autoxidation products of linoleic acid (LA) were analyzed, when the weight became 1. 14-fold under the autoxidation conditions of satisfactory atmospheric oxygen, at 37°C, in the dark, for 7 days. The LA absorbed 2.8mol of oxygen to form secondary degradation products. This autoxidized LAconsisted of 45%intact substance, 22%a mixture of polymers and endoperoxides, 18% LA hydroperoxides, 3% polar products, 1.7% azelaldehydeic acid, 1.3% hexarial, 0.9% azelaic acid, 0.6% octanoic acid, 0.3% suberaldehydeic acid, and so on. Thus, unstable 2,4-dienoic carbonyls were the main intermediate products of autoxidation of LA. Therefore, malonaldehyde was not a main product nor a major thiobarbituric acid reactive substance. Linoleic acid (LA) reacts easily with atmospheric oxygen to form linoleic acid hydroperoxide (LAHPO). The LAHPO is broken down in the presence of oxygen1} and subsequently gives a mixture of various kinds of further oxidized products. The mixture is generally termed secondary degradation products (SP). The autoxidation mechanism involving photosensitized oxidation has been gradually clarified by some laboratories,2~6) and several components of SP have been identified by a couple of workers.7~9) These autoxidation products, both LAHPO and SP, are deleterious to animals. It is considered that their toxic effects on animals are due to the sum total of many kinds of products,1^such as LAHPO,11} hydroperoxyl alkenals,12) aldehyde compounds,13~15) malonaldehyde (MA),16) and so forth. These toxicants are contained in our daily food, al-14C]LA was diluted with the cold LA to a specific activity of 51.4 kBq/mmol (1 kilo Becquerel=27 nano Curie). Alka-2,4-dienals were purchased from Aldrich Chemicals Co., Inc. Autoxidation. The LA was autoxidized at 37°C in the dark by the following three procedures: "Closed," "Open" and "Rotating." Under the "Closed" conditions, 25 g of LA was put into a small petri dish ((j> 75mm) and kept covered. Under the "Open" conditions, 5g of LA was placed in a large petri dish (> 120 mm)without a cover and