Abstract.It has previously been established that 20–100 mg of polyunsaturated oils given daily to young rats raised on a 1 % cholesterol—10 % hydrogenated coconut oil diet results in a pronounced reduction of the serum cholesterol concentration. By means of extraction of the serum lipids, chromatographic separation, and alkali isomerization it has now been shown that this decrease in serum lipids, reflected in reductions in both free and esterified cholesterol and in non‐cholesterol lipids, is accompanied by characteristic changes in the polyenoic fatty acid composition of the cholesterol esters. Linoleste supplementation resulted in the appearance of more dienoic and tetraenoic acid in the esters, linolenate particularly in the appearance of pentaenoic, but also some hexaenoic acid. Cod liver oil had an effect on all polyenoic constituents, and the magnitude of this effect, referred to the actual content of such acids in the oil, was by far the strongest. This indicates a preference in the organism for the ready made polyunsaturated acids. The largest reductions in total cholesterol were correlated with a preponderance of lino‐lenic acid‐type unsaturation in the esters, a fact which emphasizes the relative inferiority of linoleic acid in the aspect of cholesterol metabolism her