Duration of torpor by eastern chipmunks, Tamias striatus, kept at 5°C progressively increased at the beginning of hibernation (autumn phase), remained long and relatively constant (winter phase), and then decreased (spring phase) in a fashion similar to that observed in hibernators that store energy as body fat. Duration of bouts of torpor in winter and spring and amount of time animals hibernated before starting the energetically demanding spring phase were related inversely to amount of food provided at the beginning of the experiment. In addition, chipmunks given the least amount of food (400 g) continued to hibernate until they consumed all seeds; those provided with abundant food (5,000 g) terminated hibernation spontaneously early in the spring phase and remained euthermic thereafter. Animals given an intermediate ration (1,000 g) hibernated until food was gone (42%) or spontaneously terminated hibernation with abundant food uneaten (58%). Thermoregulatory responses of chipmunks appear more flexible than those of species that store energy as body fat, but the match between use and availability of energy during dormancy is imprecise.