During a 2-year period, radiotelemetry was used to continuously monitor body temperature ( T b) of free-ranging woodchucks (Marmota monax) in southeastern Pennsylvania. Hibernation was preceded by daily T b fluctuations ("test drops") of 2-4°C. During hibernation, woodchucks exhibited the characteristic pattern of torpor bouts. Time of arousals occurred randomly, but onset of torpor occurred predominantly between 1800 and 0000 h. Males had shorter hibernation periods (mean of 104.8 days) than did females (121.8 days). Males had shorter torpor bouts, but euthermic bouts were the same length as in females. Males also maintained higher T b during torpor. Overall, the cost of hibernation was greater for males than for females: males spent 38% more energy than did females. The primary energetic expense for both sexes was the periodic maintenance of euthermy throughout hibernation, which accounted for 75.2% of the energy budget for males and 66.8% for females. Compared with the 1999-2000 hibernation seasons, woodchucks during the 1998-1999 season had longer euthermic bouts, fewer torpor bouts (11.8 compared with 13.1), and spent less time in torpor (68% compared with 75%). These differences conserved more energy during the 1999-2000 hibernation season and may have been the result of severe drought conditions during summer 1999. After emergence from hibernation, woodchucks generally maintained a constant state of euthermy throughout the active season, with T b fluctuating daily by 1-2°C. However, during the summer drought of 1999, daily T b fluctuated 8-17°C in 5 of 8 woodchucks, presumably to conserve energy and water.