The distribution and histological appearance of brown adipose tissue in the newborn infant were studied. The main brown adipose masses are situated deeply, in the mediastinum or abdomen, but important masses are present in the interscapular, axillary and cervical regions. Immature brown adipose sites may be identified in pre-viable infants; such sites probably already contain their full complement of adipose cells. As the infant matures to term a large increase occurs in the volume of cytoplasm per cell, and this is accompanied by increasing granularity due to the multiplication of mitochondria. There is a progressive fall in the fat content of brown adipose tissue in the first few days of life in infants examined post mortem. It is greater in premature than in mature infants. The infants of diabetic mothers maintained a high fat content over this period. Total depletion of brown adipose tissue was found in babies dead of the cold syndrome and in 2 elderly adults who were admitted with a diagnosis of hypothermia. Reasons are given for regarding brown adipose tissue not as a phase in the development of white adipose tissue but as a mature specific tissue the function of which is the production of heat.