The impairment of brown adipose tissue (BAT) thermogenic activity with aging has been well documented. The current study investigated the effect of swimming training on BAT activity in 2-month-old (young) and 26-month-old (old) male mice. The trained mice underwent a 6-week swimming program (1 h/day, 5 days/week) in water at 35-36 degrees C. Compared with young sedentary mice, the BAT-to-body mass ratio was markedly smaller in old sedentary mice, accompanied by the decreased amount of protein, whereas there was no significant difference in uncoupling protein (UCP) content, UCP mRNA expression, or guanosine 5'-diphosphate (GDP) binding (an index of UCP activity) between young and old mice. Meanwhile, the swimming training definitely increased BAT mass and its protein content in both the young and old mice, suggesting hypertrophy and hyperplasia. In addition, after the swimming training, the amounts of protein, UCP antigen, and GDP binding in the mitochondria recovered from BAT of both mice increased significantly as compared with the respective sedentary groups, while the expression of UCP mRNA did not vary substantially. These findings suggest that, irrespective of age, swimming training enhances the thermogenic activity and capacity in BAT of mice.