In this paper, the influence of dietary fat alteration and level upon digestibilities of total fat, unsaponifiable matter and non-altered fatty acids, is studied. 11 groups of Wistar rats were fed diets containing non-heated and thermoxidized olive oils. Analytical techniques previously studied were used for isolation and evaluation of non-absorbed lipids present in feces. The results obtained led to the following conclusions: - Digestibility of fat decreases with dietary fat alteration but it is independent of the dietary fat level. - The apparent absorption of non-altered fatty acids is affected by the dietary fat alteration. - Dietary fat alteration significantly contributes to increase excreted levels of unsaponifiable from endogenous sources.