Brown adipose tissue (BAT), a highly thermogenic tissue in young animals, is relatively atrophied and thermogenetically quiescent (e.g. as measured by colonic temperature) in mice that are obese or old. The aim of the current study was to investigate the effect of aging (3.1 (young) versus 14.6 (old) months old) on BAT activity in lean and obese (ob/ob) mice. In young but not in old mice, BAT mass in terms of weight per unit body weight was significantly lower in obese mice than in lean mice. A significant increase in BAT mass of obese mice with age was noted in terms of weight or weight per unit body weight, probably because of a tendency to become white adipose tissue and the deposit of fat, accompanied by the lowest levels of total protein, guanosine 5'-diphosphate binding, and uncoupling protein (UCP) antigen in the mitochondria of BAT, as well as the lowest colonic temperature among the groups examined. Unlike old lean animals, the old obese (ob/ob) animals did not increase but rather decreased the expression of mRNA for UCP in the mitochondria of BAT. These findings suggest that a marked decrease in BAT thermogenic capacity and activity is noted in old obese mice, probably due to synergism of aging and obesity.