We report on FMR of free-living American martens (Martes americana) in autumn and winter in northern Wisconsin. Mean body mass was significantly higher in males (1099 ± 43 [S.E.] g) than females (737 ± 28 g), with no significant difference by season. Daily mass change rates of martens did not differ from zero, and mass change rate and percent of body fat did not differ by gender or season. These data are consistent with our expectation that non-reproductive martens balance their energy budget on a near daily basis, even in winter, and rely little on body energy reserves. Energy expenditure, and hence food requirements, declined 24% from fall (1006 ± 96 kJ/d) to winter (725 ± 96 kJ/d), despite colder temperatures and deep snow. Both males and females were active nearly 50% less in winter (4.8 ± 1.0 h/d) than in autumn (9.1 ± 0.7). It appears that, in addition to lowering core body temperature and seeking thermal cover, martens decreased activity levels from fall to winter to reduce energy expenditures and food requirements.