1. In 125 cases of perirenal fat samples derived from human necropsies (from 1 month to 86 years), the occurrence of the brown adipose tissue was examined. Brown fat cells were contained in 72% (90: 125) of the cases. 2. In the infants, all samples contained maximal amounts of brown fat cells: the perirenal fat was composed almost exclusively of brown fat cells: in children and teenagers they began to diminish, and in younger adults further reductions were observed: after the fifth decade conspicuous diminutions occurred. In later decades the brown fat in the perirenal fat was small in amount. 3. This change in the amount of the brown fat tissue undergoes individual variations. In some cases, the brown fat tissue disappeared from perirenal fats in the early stages of life, while in others it persisted to very late stages of life. A man as old as 86 years possessed brown adipose cells in the perirenal fat. 4. Multilocular brown fat cells were classified into the following types: Type 1, fat-depleted cells: Type 2, small-locular cells: Type 3, middle-locular cells: Type 4, large-locular cells: Type 5, monolocular brown fat cells with a thick cytoplasmic rim and pseudomonolocular brown fat cells and Type 6, multilocular brown fat cells rich in cytoplasm. In the infants, all cell types were identified: the small-locular cells were in general scanty in all decades following infancy: in later decades of life, the most common cell types were middle-locular and large-locular cells. The fat-depleted cell is a particular cell type and may occur usually accompanied by multilocular brown fat cells rich in cytoplasm. 5. The fat lobules are composed of a centrally located brown fat cell area and a peripheral layer of monolocular white fat cells, which in the infants was very thin but in the following ages was gradually widened to invade the interior of the lobules. 6. The present findings suggest a continuous replacement of brown fat cells by white fat cells during advancing age. The monolocular brown fat cell with a thick cytoplasmic rim and the pseudomonolocular brown fat cells probably represent transitional forms between multilocular brown fat and monolocular white fat cells. These cell types were found throughout life, suggesting continuous transformation of the brown fat cell into the white fat cell.