Previous researches have shown the importance of natural antioxidant substances for prevention of formation or removal of free radicals, which are thought to be the trigger involved in diseases such as cancer, heart syndromes and problems linked to the ageing process. In the present work, the effects of extra-virgin olive oil and ethanol, associated with selective diets provided to laboratory rats, have been studied in order to evaluate their relative effects on cellular oxidative damage. The effect of olive oil have been compared to those of butter and soya oil. The levels of oxidised cellular constituents, such as proteins and lipids, as well as the oxidation of glutathione and vitamin E in mitochondria have been studied