Young adult male rats were given a portion (42%) of their normal daily food intake by stomach tube (tube-fed), but they were also allowed free access to the same powdered diet. Total metabolizable energy intake was identical to that of controls, but tube-fed rats gained 25% weight and 50% more body energy than controls over 15 days. Total energy expenditure and resting oxygen consumption, measured during the latter part of the day, were reduced in tube-fed rats. Intubated animals showed a lower thermogenic response to noradrenaline and depressed activity of the mitochondrial proton conductance pathway in brown adipose tissue (assessed from GDP-binding), but brown fat mass, protein content, the density of beta-adrenoreceptors in brown fat cell membranes and plasma triiodothyronine (T3) levels did not differ between groups. These data indicate that the enhanced energetic efficiency and fat deposition of tube-fed rats is due to a lower energy expenditure, resulting, at least in part, from lower levels of thermogenesis in brown adipose tissue.