The brown adipose tissue of the hibernating hedgehog lies between the neck and the back musculature. It is most abundant and most deeply colored in October, and gradually disappears during hibernation. Histological and chemical studies have shown that this tissue differs in structure, composition, etc., from ordinary fat tissue. Hibernating hedgehogs were killed in Jan., and the brown fat extracted with 80% acetone. After standing in the ice box, a dark fraction, which contained the active principle, separated from the concd. acetone soln. The effect of the active principle on the basal metabolism of rats was detd. with the Zeiss laboratory interferometer, the O2 deficit and CO2 production being measured. After the normal basal metabolism of rats was detd., 0.5-0.8 cc. of the active extract was injected into the abdominal cavity. Within 2-6 hrs., there was a marked lowering of the basal metabolism, which persisted for 24 hrs. or more. The injection caused a fall in body temp., and an increase in the sugar and Mg concs. of the blood. Acetone extracts of ordinary fat did not produce these results. Injection of 0.8 cc. of the active extract into rats which had been injected with 1 mg. thyroxine per kg. body weight for 5 days, lowered the basal metabolic rate and usually resulted in death. One group of rats was injected with 3 mg. thyroxine/kg. body wt. for 4 days, and a 2d group with 100 units prolan (gonadotrophic principle of the ant. hypophysis) daily for 6 days and with thyroxine. The prolan suppressed the increase in the basal metabolic rate caused by the thyroxine.