The ability to regulate body temperature diminishes with age. Exercise training is known to increase cardiovascular performance, and there is some evidence of a cross-adaptation between exercise and cold tolerance in young rats. The present study was designed to examine the effects of physical training by treadmill running on the capacity for brown adipose tissue (BAT) thermogenesis in young and old rats. To this end, we assessed BAT uncoupling protein (UCP) mRNA expression in sedentary and exercise-trained 5- and 25-mo-old F-344 rats. The amount of UCP mRNA, whether expressed as per unit RNA or per BAT, did not change with either age or training. These data indicate that there is no cross-adaptation by exercise on adaptive thermogenesis in BAT in either young or old rats.