Changes in total weight, protein content and activity of cytochrome oxidase have been followed in the brown adipose tissue (BAT) of golden hamsters, white mice and white rats adapted to various temperatures. Important interspecies differences have been detected. The weight and the cytochrome oxidase activity of the BAT of the white mouse changes little under the influence of different adaptational temperatures, whereas in the white rat and in the golden hamster a decrease of adaptational temperature considerably increases the total weight, protein content and cytochrome oxidase activity of this organ. Different adaptational temperatures induce variable effects on the BAT of the golden hamster. Mild cold stimuli favour the accumulation of proteins and of oxidative enzymes, in particular. Severe cold stress also induces growth processes, so that the weight of the BAT increases proportionally to the total body weight of animals. The metabolic capacity of the BAT, measured as total cytochrome oxidase activity, changes substantially with decreasing temperature of adaptation in all species studied, increasing twice, six times and almost twelve times in the mouse, rat and golden hamster, respectively.