Seasonal acclimation of nonshivering thermogenesis and brown adipose tissue was studied in wild bank voles (Clethrionomys glareolus), yellow necked field mice and wood mice (Apodemus flavicollis, A. sylvaticus). Both, voles and mice increased their capacity for nonshivering thermogenesis during winter. Thermogenic properties of brown fat (cytochrome c oxidase activity, mitochondrial protein content, GDP-binding of brown fat mitochondria) showed similar changes during seasonal acclimation; Clethrionomys and Apodemus spp. both showed lowest thermogenic properties in the summer during August, a rapid increase during fall, and highest levels of thermogenic activity in the winter months. With regard to changes in body weight and brown fat mass these species show different strategies for seasonal acclimation. In Clethrionomys a reduction of body mass in the winter was found, both in the wild population as well as in individual animals housed in the laboratory. A. flavicollis showed a reduction of body weight during fall, whereas A. sylvaticus maintained a constant body mass throughout the year. Brown fat mass and cellularity increased in the Apodemus spp. during winter, in parallel with the thermogenic properties of brown fat, whereas in Clethrionomys brown fat mass and cellularity remained seasonally constant. These species live in the same habitat and were trapped in the same area. It is concluded that seasonal improvements of in vivo and in vitro thermogenesis are very similar in these species, although the physiological basis for this improvement is different in Clethrionomys and Apodemus.