Rats were chronically acclimated to 28 degrees C or 5 degrees C or submitted to daily variations of ambiant temperature. Ten or thirty days after removal of about 40% of the total brown adipose tissue (whole interscapular and 25% of abdominal tissues), the weight of the different pads of brown adipose tissue, thyroid and adrenals were determined. In all the groups, there was a large decrease of brown adipose tissue weight for the first ten days due to the shock following the operation. Then, the brown adipose tissue weight was restaured and, only in constant cold accliclimated rats, compensative hypertrophies of axillary and thoracic brown adipose tissue were found. Adrenals weight was significantly increased after the operation; in the two groups of cold acclimated rats, that increase was still significant one month later. However, the corticosterone production rate was not increased. These results are discussed in relation to the physiolocical role of brown adipose tissue in cold acclimated animals.