The effect of cold-adaptation was investigated on the brown adipose tissue of Djungarian hamsters. Animals maintained at 23 degrees C and 16 hours light per day (controls) were exposed to 5 degrees C. The wet weight of the total brown fat is reduced by some 40% within 4 days of cold-exposure, as a result of extensive triacylglycerol depletion of the tissue with no reduction in DNA; the tissue mass remains constant under persistent cold influence. The total amount of tissue mitochondria is doubled by 24 h and increases by a factor of 3 under persistent cold-stimulus, the specific respiratory capacity of the organelles remaining unchanged. The amount of 32 kDa regulatory protein per mg mitochondrial protein quantified from high-affinity GDP-binding, is increased by a factor of 2.7 after 21 days of cold-adaptation; a 9-fold increment is found of the total mitochondrial GDB-binding capacity. Comparison of nonshivering thermogenesis and the maximal thermogenic capacity of brown fat, estimated from the maximal respiration of the isolated mitochondria and the total amount of mitochondria in the tissue, suggests that brown fat may contribute about 20% to the whole-body nonshivering thermogenesis in warm-adapted controls and 45% in cold-adapted hamsters. The estimated increase in thermogenic capacity of the tissue in response to 21 days of cold-adaptation corresponds to the increase in nonshivering thermogenesis, suggesting a central thermoregulatory role of brown fat during cold-adaptation.