Summary12-Keto oleic acid, possibly one of the oxidation products of long-chain, unsaturated fatty acids, was added to the feed of weanl ing male rats at the 1% level. Their growth curves, tissue weights, plasma alkaline phosphatase, GOT, and GPT activities, and plasma and liver lipid (cholesterol, triglyceride and phospholipid) levels were investigated and compared with those of weanlings fed a vitamin E deficient diet. Both the diet containing 12-keto oleic acid and the diet deficient in vitamin E decreased the growth rate of body weight and tissue weight, and increased the liver triglyceride and cholesterol levels. Parallel with these, increased hemolysis and stimulation of lipid peroxidation and fluorescent production in the liver homogenate were observed. Elevated plasma alkaline phosphatase and GOT activities which may be considered to be due to a functional disorder of the liver were also observed.