We have previously shown that after severe cold stress of rats the epididymal white adipose tissue (WAT) acquires many of the morphological characteristics (innervation, vascularization, appearance of adipocytes) of brown adipose tissue (BAT). In the present study, the mitochondrial characteristics have been analyzed. Mitochondria from the epididymal fat pad of cold-stressed rats became enlarged and had a volume 1.7 times greater than that in the control group (0.5 microns 3 in the experimental versus 0.3 microns 3 in the control group). The mitochondria occupied about 60% of the cytoplasm (14% in the control group), thus approaching the highest value reported for any cell. The straight or slightly wavy cristae completely traversed the width of the mitochondria and occupied an area of 26 microns 2/1 microns 3 of mitochondrion in the experimental adipose tissue (7 microns 2 in the control group) and 15 microns 2/1 microns 3 of cytoplasm (1.4 microns 2 in the control). The mitochondrial matrix became electron lucid and could contain lamellar whorls, as could the surface of the mitochondria. These mitochondria of epididymal WAT resembled, both morphologically and morphometrically, mitochondria in the BAT of cold-exposed rodents. In the epididymal mitochondria, immunoelectron microscopy did not reveal the presence of the BAT-specific uncoupling protein thermogenin, nor could the slot-blot technique detect thermogenin mRNA. We conclude that even under these extreme conditions of cold stress, WAT cannot adopt the thermogenin-dependent thermogenesis of BAT. The dramatic mitochondriogenesis can be interpreted only as being indicative of an extremely high metabolism in the tissue, thus placing unprecedented pressure on the energy turnover capacity of the cell.