The effects of fasting and refeeding on nonshivering thermogenesis and the properties of brown adipose tissue have been investigated in mice. Fasting for 48 h led to a substantial reduction in the capacity for nonshivering thermogenesis, and there was no recovery of thermogenic capacity during the first 5 days of refeeding. A period of 10-15 days of refeeding was required for full restoration of thermogenic capacity. The mice were hyperphagic during the first 6 days of refeeding, but body weight was recovered after 24 h. The amount of interscapular brown adipose tissue decreased substantially on fasting, but it recovered 24 h after the initiation of refeeding. Cytochrome oxidase activity, the level of mitochondrial GDP binding, and the specific mitochondrial concentration of uncoupling protein in brown adipose tissue were each reduced by fasting. Although both GDP binding and the specific concentration of uncoupling protein rapidly returned to normal on refeeding, the activity of cytochrome oxidase was not normalized until 10 days after the end of the fast. These results indicate that a prolonged period of refeeding is required for the recovery in the capacity for nonshivering thermogenesis following a fast, a similar time course being evident for the recovery of cytochrome oxidase activity in brown adipose tissue. It is suggested that the fasting-induced reduction in the capacity for nonshivering thermogenesis is linked primarily to a loss of mitochondria from brown adipose tissue and that the normalization of thermogenic capacity is dependent on the restoration of mitochondrial mass.