Used frying oil can be separated by means of column chromatography on silica gel into an unpolar fraction, which contains predominantly the unaltered tri-glycerides, and a polar fraction consisting of all oxidation and decomposition products formed during the heating process. The size of the polar fraction indicates the degree of fat deterioration. A similar procedure was applied to obtain fractions from a heated oil to be used in long-term feeding experiments. Several tons of a sunflower oil which had been used in the industrial production offish fingers were separated into a polar fraction(l) and an unpolar fraction (2). The sunflower oil had not been overheated and was taken at the moment when the production would have been stopped, according to factory practice, and the oil discarded. Fractions 1 (group U) and fraction 2 (group P) as well as the original unheated sunflower oil (group F) and the heated sunflower oil (group H) were fed to rats over 18 months at a level of 20% in the diet. Fraction 1 caused a highly significant reduction in weight gain of the animals as compared with unheated sunflower oil but had only an insignificant detrimental effect upon the many biochemical, histological and clinical parameters. The order of the weight gain caused by the four samples was: U < H < P < F. The changes of other parameters as well as the implications of these long-term feeding studies will be discussed in detail.