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The low body temperature state of hibernating mammals is interrupted regularly by short euthermic phases. The frequency of these euthermic phases changes according to the phase of hibernation. This typical pattern, common among ground squirrels (genus Spermophilus F. Cuvier, 1825), can be regulated by internal factors and affected by outside temperature. To evaluate the effects of internal annual timing and ambient temperature, we monitored nest temperatures of overwintering European ground squirrels (Spermophilus citellus (L., 1766)) under three temperature conditions during late hibernation (9, 5, 0 °C). Our results showed that in spite of constant thermal conditions, an annual timing effect changed torpor-bout length (TBL) at 9 and 5 °C but not at 0 °C. Ambient temperature had an independent influence on TBL and euthermic phases. The loss in body mass was higher at higher ambient temperatures …