In 134 out of 180 perirenal fat samples (74%) derived from Japanese necropsy cases aged from 1 month to 86 years, the brown fat tissue persisted in variable amounts. Brown fat cells were classified into 6 types: Type 1 cells are fat-depleted cells filled with granular cytoplasm and are believed to be produced after oxidation of fat for heat production. Type 2 cells are small-locular cells suitable for rapid oxidation of fat droplets. Type 3 (middle-locular) and 4 (large-locular) represent fat-storage cells containing large amounts of fat. Type 5 cells are thought to be transitional forms between multilocular brown fat cells and monolocular white fat cells. Type 6 (cytoplasm-rich multilocular) cells, usually found together with Type 1 cells, are thought to be fat-depleting or -consuming cells, since in them fat droplets are reduced in number and size probably in consequence of oxidation of fat, but by contrast granular cytoplasm is increased in amount separating the individual fat droplets by thick cytoplasmic septa. The occurrence of Types 1 and/or 6 cells that has been revealed in 65 out of the total 180 samples (36%), suggests that the oxidation of fat for the thermogenesis proceeds in the brown fat tissue and that brown fat cells partially undergo fat depletion. In the present study, the thermogenesis of human brown fat tissue was suggested chiefly with regard to the occurrence of Types 1 and/or 6 cells. In the majority of perirenal fat samples from infants (1-11 months) relatively numerous Types 1 and 6 cells were frequently revealed together with Type 2 cells, suggesting rapid and active heat production in support of the view that in human infants the brown fat tissue may be thermogenetically active to maintain body temperature. In the same manner, marked ability to produce a considerable amount of heat was evidenced in brown fat tissue of children and teenagers. In younger and elderly adults the frequency of occurrence and the amount of the perirenal brown fat tissue were decreased but Types 1 and/or 6 cells could be found in 17-40% of them, infrequently together, with Type 2 cells, suggesting persistence of the thermogenic activity with occasional large heat production especially in younger adults (20-39 years). Thus, the results obtained in this study have clarified that the human brown fat tissue can respond to stimuli given to the body by oxidation of stored fat even in the latest decades of life. In cases of death from burning, drowning, bleeding, drug poisoning etc., numerous Types 1 and/or 6 cells were found, suggesting that an active fat oxidation would take place in brown fat tissue assumedly as the result of the raised noradrenalin level in this tissue. The so-called small cytoplasmic cells found in perirenal fats from cases of death from liver cirrhosis and other causes were assumed to be atrophic fat-depleted brown fat cells.