We have found that cold- and norepinephrine-induced brown adipose tissue (BAT) nonshivering thermogenesis (NST) is significantly lower in old male Fischer 344 rats and is associated with the decreased ability of these animals to maintain homeothermy. This decline in BAT thermogenesis is not as great in females. Although the mechanism(s) underlying this gender difference in the age-related decrease in brown fat NST are not completely elucidated, they do not appear to reflect decreased sympathetic neural activity of BAT in the older males vs. females. Rather, our investigations, strongly suggest that the blunted cold-induced heat production of BAT reflects less functional BAT. The fact that the older animals have less functional BAT than do their younger counterparts may predispose them to the accumulation of excess body fat. Our studies have also found that near the end of the natural life of these rats, they enter a state of senescence that can be identified by spontaneous rapid body weight loss, resulting from decreased food intake. In this state, the rats are considerably more susceptible to cold than are comparably aged presenescent (body weight stable) rats of the same chronological age. The greater hypothermia exhibited by the senescent vs. presenescent rats during cold exposure is associated with a significant reduction in the amount of functional brown fat and in the amount of heat each brown fat cell can generate. It is the intent of this review to discuss the findings of these investigations.