Measurements of energy balance, thermogenic responses to noradrenaline and brown adipose tissue (BAT) activity were performed in male Lister-hooded rats aged 3.5 and 6.5 months, and fed either a pelleted control diet or a palatable cafeteria diet for 15 d. Cafeteria feeding produced increases in energy intake of 34 and 30 per cent in 3.5 and 6.5-month-old rats respectively, and energy expenditure was elevated by 25 and 10 per cent in these groups. Three-and-a-half-month-old cafeteria-fed rats gained more energy than their controls, but net energetic efficiency was significantly reduced, while in the older cafeteria rats, body energy gain was markedly increased without any apparent effect on net efficiency. The thermogenic response to noradrenaline was enhanced by cafeteria feeding at both ages. The younger cafeteria-fed rats showed significant increases in the mass, protein content and mitochondrial yield of BAT, and the activity of the mitochondrial proton conductance pathway, assessed from GDP-binding, was greater than their controls. The 6.5-month-old cafeteria group also showed hypertrophy of BAT and small, but not significant, increases in the protein content of the tissue and mitochondrial GDP-binding. These results demonstrate that rats aged 3.5 months can exhibit diet-induced thermogenesis and activate BAT in response to overfeeding, but the capacity for thermogenesis declines with age and was virtually absent in 6.5-month-old rats.