We undertook a study to determine presence of circadian rhythms during woodchuck hibernation using continuously monitored body temperatures. Males had shorter torpor and longer euthermic periods than females. Circular statistics revealed a significant mean vector for males entering into torpor (10:21 h), but not for females. No significant mean vector was found for male or female arousal from torpor. A contingency test was applied to the torpor bout durations. All 7 males tested had significant tau's between 24 and 26 h, while 6 of the 13 females tested had significant tau's with a range of 22-27 h. These results implicate a free-running circadian clock during torpor bouts. Overall, the data support the existence of biological rhythms during hibernation in woodchucks, especially for males during arousals. Since entries into torpor appear to be synchronized for males, arousal periods may be used to resynchronize their circadian system. The persistence of biological rhythms during hibernation may help to insure successful mating in the spring after emergence.