Homeothermic animals must maintain their body temperatures within a very narrow range. Large homeotherms face problems with dissipation of metabolic heat: for small homeothermic animals heat losses at most environmental temperatures are far in excess of normal metabolic heat production. Accordingly, brown adipocytes exhibit in mammalians a very rapid process of morphological functional maturation perinatally. This thermogenesis occurred without electrical activity in the skeletal muscle and it is therefore termed ‘nonshivering thermogenesis’. The anatomical site of nonshivering thermogenesis is mainly the brown adipose tissue in the small newborn mammals. In animals reared under thermoneutral conditions the tissue subsequently involutes and nonshivering thermogenic capacity is largely lost. Cold- and diet-induced adaptation may slow or prevent this involution. The decline in the extent of nonshivering thermogenesis during development at thermoneutral temperatures is reflected by an alteration in the bioenergetic properties of the isolated brown fat mitochondria.