The attenuated thermogenic responses observed in older animals exposed to low environmental temperatures may reflect decreased thermogenic capacity of brown adipose tissue, a major nonshivering thermogenic effector in rats, and/or decreased metabolic capacity of lean body mass. To evaluate these possibilities, the oxygen consumption of female Sprague-Dawley rats aged 5 and 26 months was recorded at rest and during 6 h of exposure to 6 degrees C. The thermogenic capacity of brown fat was estimated from the binding of guanosine-5'-diphosphate (GDP) to isolated brown fat mitochondria. Both resting and cold-induced oxygen consumption expressed on a mass independent basis [ml/(min x kg body mass.67)] and as a function of lean body mass [ml/(min x g lean body mass)] were significantly lower in the 26-month-old animals. Colonic temperatures of younger and older rats after the 6 h of cold exposure were 37.5 +/- 0.1 and 36.1 +/- 0.2 degrees C, respectively, and were significantly different. However, no significant differences in the binding of GDP to isolated brown fat mitochondria were observed. These data indicate that the thermogenic capacity of brown fat is not decreased in the aged rat, and that the metabolic capacity as well as the amount of lean body mass is altered with age.