The depot fat of the rat contains 1.5% of C20 and C22 unsaturated fatty acids; that of a tortoise (Testudo graeca) contains 7%, and that of a lizard (Varanus salvator) contains 5%. Comparison of these values with those obtained by the author in the brown grass frog and by others in freshwater fish indicates that the degree of unsaturation depends on the body temperature of the animal. It is suggested that the highly unsaturated C20 and C22 fatty acids are absent from the depot fat of warm blooded animals because in their more active metabolism these fatty acids are quickly oxidized and, accordingly, cannot as such be deposited.