Presenting rats with a 0.9 per cent sodium chloride solution to drink instead of water had little or no effect on body weight gain and food intake, but resting oxygen consumption and total energy expenditure (corrected for body size) were elevated, and thermogenic responses to both noradrenaline and a meal were enhanced. Brown adipose tissue (BAT) mass and protein content were significantly elevated in saline treated rats, but mitochondrial GDP-binding capacity was depressed. Basal Na+, K+-ATPase activity was slightly increased in BAT homogenates from rats given saline, but noradrenaline-stimulated enzyme activity was much greater than control values. In rats drinking 1.8 per cent saline, energy intake, body weight gain and the efficiency of gain (g gain/MJ eaten) were all markedly depressed. BAT mass, corrected for differences in body size, was slightly greater than controls and the protein content of BAT was increased by 45 per cent. Rats allowed 0.9 per cent saline to drink for 7 d, and then presented with a palatable cafeteria diet, showed a more rapid rise in metabolic rate than cafeteria-fed animals drinking water. This difference was apparent only over the first 3-4 d of cafeteria feeding, and energy balance over 14 d was similar for both groups. These data show that increasing sodium intake with isotonic saline has very little effect on food intake or resting metabolic rate, but causes a marked increase in thermogenic capacity and responses to food or noradrenaline, probably because of an increase in active BAT mass. Changes in plasma ion concentrations or osmolarity, therefore, could be involved in the thermogenic response to food.