The duration of the hibernation season of 13-lined ground squirrels, Citellus tridecemlineatus, repeatedly aroused over the winter, was only slightly less than that of undisturbed animals. The aroused animals ate more than eight times as much food during the middle of their hibernation season. This suggests that ground squirrels compensate by increased food intake for extra energy expenditure forced on them by repeated arousals. Aroused animals deprived of food lost weight at a greater rate than undisturbed animals. When food was returned they gained weight rapidly. However, weights did not return to predeprivation levels, but only to levels appropriate for the stage of the cycle at the time food was returned. These findings suggest that, even when ground squirrels are losing weight over the winter, they regulate their weight, but at a level that progressively declines. The underlying mechanisms probably depend on adjustments to the balance between medial and lateral hypothalamic areas.