During arousal from hibernation, body temperature (T(b)) increases by ∼30°C and liver mitochondrial respiration increases threefold in as little as 2 h. We analyzed liver mitochondria purified from ground squirrels (Ictidomys tridecemlineatus) to see whether membrane phospholipids were remodeled during spontaneous arousal. Cardiolipin content did not change among animals in torpor (T ∼ 5°C), the early phase of arousal (T ∼ 15°C), late arousal (T ∼ 30°C), interbout euthermia (T ∼ 37°C), and summer-active animals (T ∼ 37°C) that do not hibernate. Phosphatidylcholine content increased in late arousal relative to interbout euthermia, while phosphatidylethanolamine decreased. Phospholipid monounsaturated fatty acids (MUFAs) did not change throughout arousal, but polyunsaturated fatty acids (PUFAs) and MUFA/PUFA decreased and increased, respectively. In the fatty acid conjugates of phospholipids, neither unsaturation index nor n-3/n-6 differed. Few changes in individual fatty acids were noted, but palmitoleic acid (16:1, n-7) was higher in interbout euthermia and summer. Although 16:1 accounted for less than 1.5% of phospholipid fatty acids, it correlated strongly and positively with succinate-fueled state 3 mitochondrial respiration. No other phospholipid characteristic measured here correlated with mitochondrial respiration. These data show that mitochondrial membranes are remodeled rapidly during arousal, but the contribution to reversible suppression of mitochondrial respiration remains unclear.