This review deals with the comparative observations of brown fat tissue in the bat, a hibernator, and in man, a nonhibernator. In both mammals, the brown fat is distributed in restricted portions of the body and brought into a thermogenetic activity by an acute drop in ambient temperature. Light microscopic examination was performed on the interscapular brown fat of bats captured monthly throughout one year; electron microscopic observations were made using a bat captured in April and another in September. Human perirenal brown fat was investigated light-microscopically on tissues derived from 215 fresh necropsy cases (Japanese) of both sexes aged from one month to 93 years. Brown adipose tissue was recognized only in 162 (75%) of the 215 samples, because brown fat cells were reduced by transformation into white fat cells. Human brown fat cells were classified into six types according to the morphological features of their lipid droplets. These reflect different functional states of intracellular heat production. The Type 1 (D) cell is a fat-depleted brown fat cell with a darkly stained cytoplasm; it is specific to humans. Human perirenal brown fat cells begin to show a transformation into white fat cells already at the infantile stage. This change occurs from the peripheral toward the central portion of the lobule, so that various functioning cell types remain only in the central area of the lobules. In contrast to humans, bat interscapular brown fat cells exhibit regular seasonal changes in size and lipid droplet content, so that the cells could not be classified as in humans into definite types according to the features of their lipid droplets. The most conspicuous difference between brown fat tissue in the nonhibernator and hibernator is that in the latter, white fat cells never occur within the brown fat tissue. In the hibernator, thermogenesis in the brown fat is necessary for both the arousal from hibernation and the maintenance of hibernation as well as rutting. In human perirenal brown fat tissue, darkly stained fat-depleted cells (D) occupy, with other cell types (CR, CR'), an important part in the reversible heat production cycle of the brown fat tissue. The "hypothermic" or "cold syndrome (cold injury)" is a disorder affecting inadequately protected infants in severely cold seasons, accompanied by lethargy, hypothermia and lactation refusal and revealing hemorrhagic pneumonia in necropsy. The brown fat tissue in such infants is composed exclusively of fat-depleted brown fat cells.(ABSTRACT TRUNCATED AT 400 WORDS)