It is known that carp fed autoxidized oil reveal poor growth, high mortality and characteristic muscular dystrophy called “ Sekoke ” disease. But it has not been clarified as to what is the main toxic substance in autoxidized oil. To determine the exact effect of autoxidized products, pure methyl linoleate hydroperoxide and secondary decomposition products were prepared, and a feeding experiment was carried out using carp for 120 days. The results indicate that both of these autoxi-dized oils induced muscular dystrophy, poor growth and inactivation of enzymes such as succinate dehydrogenase. The degree of their toxic effect was almost equal. In the group fed with methyl linoleate hydroperoxide, an increase of carbonyl compounds was observed in the lipids extracted from intestine and adipose tissue, although accumulation of peroxide was not recognized. These results suggest the conversion of lipid peroxide to carbonyl compounds in intestinal tissue.