1. Young male rats were fed on a pelleted stock diet or a variety of palatable food items (‘cafeteria’ diet) and housed at 24° or 29°.2. ‘Cafeteria’ feeding at the lower temperature stimulated energy intake, gain and expenditure, but reduced energetic efficiency such that over 70% of the excess intake was expended.3. Housing at 29° suppressed intake and expenditure in animals on both diets, but to a greater extent in ‘cafeteria’-fed rats and energetic efficiency was greater than control values at this higher temperature.4. The thermogenic capacity of brown fat (mitochondrial purine nucleotide binding) was increased by ‘cafeteria’ feeding, but was suppressed in animals kept at 29°.5. The results demonstrate that diet-induced thermogenesis is inhibited by high environmental temperatures.