The mitochondrial theory of aging suggests that this phenomenon is the consequence of random somatic mutations in mitochondrial DNA, induced by long-term exposure to free radical attack. There are two potential dietary means of delaying the effects of free radicals on cellular aging, i.e., enrichment of mitochondrial membranes with monounsaturated fatty acids and supplementation with antioxidants. We have performed a preliminary study on male rats, 6 or 12 month old, fed with diets differing in the nature of the fat (virgin olive oil or sunflower oil) and/or with antioxidant supplementation (coenzyme Q10), analysing hydroperoxide and coenzyme Q9 and Q10 in heart mitochondria. Preliminary results allow us to conclude that the CoQ10 dietetic supplementation as well as the enrichment of the cellular membranes with monounsaturated fatty acids, successfully protect mitochondrial membranes from aged rats against the free radical insult.