Mitochondrial protein, cytochrome-c-oxidase and mitochondrial ATPase activities, which can participate in brown adipose tissue thermogenesis, were measured in the present study in order to evaluate mitochondrial activity, oxidative capacity and ATP synthesis in dietary obese rats compared to control rats. Cafeteria-diet induced increase of cytochrome-c-oxidase and ATPase activities of 54% and 37% respectively, but mitochondrial protein content remained unchanged. Fasting induced active mitochondrial protein degradation (about 50%) only in control rats, but in both cafeteria fed and post-cafeteria obese rats fasting-induced loss of mitochondrial protein was impaired. It was concluded that cafeteria diet is able to induce specifically both the oxidative capacity and the ATP synthesis in adult rat brown adipose tissue without affecting the mitochondrial protein. Furthermore, during fasting the obese (or overweight) status 'per se' regulates the overall mitochondrial protein degradation which was impaired or inactivated in overweight dietary rats compared with controls.