To understand the basis of the thermally sensitive physiological processes that govern the duration of individual bouts of torpor, we determined torpor bout duration in relation to air temperature ($T_{a}$), body temperature ($T_{b}$), and oxygen consumption ($\dot{V}O_{2}$) in hibernating golden-mantled ground squirrels, Spermophilus saturatus, during the midst of the hibernation season when bout length was stable. Over the range of $T_{a}$ from 8 down to 2 C, length of torpor bouts increased as $T_{b}$ and $\dot{V}O_{2}$ decreased, and the difference between $T_{b}$ and $T_{a}$ (ΔT) remained about 1.5 C. However, when $T_{a}$ was lowered from 2 C to-2 C, the trend reversed and torpor duration and $T_{b}$ decreased, whereas $\dot{V}O_{2}$ and ₂T increased. This response is due to an increase in heat production that keeps $T_{b}$ from declining below about 1 C. We examined our data in terms of the hypothesis that arousal is determined by accumulation of metabolites during torpor; if this is true, then the arousal frequency should be directly proportional to energy metabolism ($\dot{V}O_{2}$). Using linear and multiple regression, we found that $T_{b}$ and $T_{a}$ are better predictors of torpor duration than $\dot{V}O_{2}$. The best relationship was, however, achieved by regression of $T_{b}$ and $\dot{V}O_{2}$. We conclude that $T_{b}$ per se also contributes, along with metabolism, to determining the length of torpor bouts.