This study investigated the way in which feeding rats with two fat sources (olive or sunflower oils) affected electron-transport components and function of mitotic (liver) and postmitotic (heart and skeletal muscle) tissues during ageing. Rats adapted the mitochondrial-membrane-lipid profile to dietary fat throughout the study, suggesting that the benefits to eat either of the two fats might be maintained lifelong. Liver was more resistant to dietary changes and ageing than heart and skeletal muscle, which showed higher levels of coenzyme Q, cytochrome b, and cytochrome a + a3 with ageing and lower cytochrome c oxidase and complex IV turnover. Dietary fat differentially modulated the response of tissues during ageing, with sunflower oil leading to the highest levels of coenzyme Q and cytochromes b and a + a3. Since high levels of cytochrome b have been related to increased age, it could be hypothesized that olive oil could lead to less aged mitochondria.