Rats fed "cafeteria" diets with low (7%, LP) normal (23%, NP) or high (33%, HP) protein contents showed increases in metabolizable energy intake (kJ/kg 0.75, 23-41%) and in energy expenditure (36%) compared to controls fed stock diet (27% protein). The high metabolic rates were inhibited by beta-adrenergic blockade with propranolol. All rats fed cafeteria diets deposited more fat than controls, but the LP diet depressed growth, and these animals also showed the lowest energetic efficiency. Brown adipose tissue (BAT) mass and protein content were increased in all groups fed cafeteria diets, but the largest changes occurred in LP-fed animals, and the smallest in the HP group. Hepatic mitochondrial alpha-glycerophosphate shuttle activity and plasma triiodothyronine levels were elevated twofold in rats fed LP cafeteria diet compared to controls, but the other cafeteria diet groups showed little or no changes, and shuttle activity in BAT was not affected by any of the diets. Blood glucose and plasma insulin levels were similar for control, NP and HP animals, whereas glucose levels were slightly lower and insulin levels were very much lower in the rats fed LP cafeteria diet.