AbstractPostnatal hamsters were examined for a metabolic response to cold by measuring oxygen consumption at ambient temperatures of 25°, 30°, 33°, and 36°C. In conjunction with this, colonic temperatures were recorded, and the animals were examined for brown adipose tissue development.During the first ten days of postnatal development hamsters increased their O2consumption linearly as ambient temperature was raised from 25° to 36°C. Thus, they are initially poikilothermic. Body temperatures were directly dependent on environmental temperature, and no mature brown fat deposits had yet developed.On days 11 and 12, O2consumption curves began to vary from a linear relation with ambient temperature, and small inflexions developed at 30° and 33°C. By postnatal days 14 and 15 maximal values were recorded at 25°C. Simultaneously, brown fat deposits developed definitive characteristics and body temperatures came to range only slightly below adult temperatures during exposure at 25°C.During the latter stage of maturation studied in these experiments, days 17 to 21, the animals achieved homeothermic stability between 25° and 36°C. Only 25°C prompted a metabolic increase, 30° to 36°C became a zone of thermal neutrality, and body temperatures came to equal or exceed adult temperatures. The animals had gained significant weight, thereby increasing thermal insulation, and brown fat deposits had accumulated ad