In winter mammalian hibernators, such as the 13-lined ground squirrel (Ictidomys tridecemlineatus), enter a state of torpor where metabolic rate is suppressed by ca. 90%, and body temperature (Tb) subsequently declines towards freezing. These changes are fully reversed during arousals, which occur spontaneously every 5-14 days throughout the winter. Arousals return animals to full euthermia for ca. 1 day before they enter another bout of torpor. This cycle is apparently driven by endogenous signals, and offers an excellent natural model to study plasticity of many physiological functions, including mechanisms of metabolic. The reversible suppression of whole-animal metabolism in hibernation is mirrored by some aspects of mitochondrial oxidative phosphorylation. In liver mitochondria isolated from torpid animals, state 3 (phosphorylating) respiration, fueled by succinate oxidation, is ca. 70% lower than in euthermic animals, even when measured at the same in vitro temperature (37oC)1. Torpid suppression of mitochondrial respiration may conserve endogenous fuels and minimize damage by ROS, but it is not evident in all tissues, including cardiac muscle and forebrain. Moreover this suppression depends on several factors, including oxidative substrate and in vitro assay temperature. For example, in mitochondria isolated from skeletal muscle, respiration is suppressed in torpor by ca. 35%, but only with succinate as substrate, and only when measured at 37oC2. Although not universal, the plasticity of oxidative phosphorylation seen in some hibernator tissues provides valuable information about the regulation of mitochondrial metabolism. In liver mitochondria suppression of state 3 respiration occurs early during entrance into torpor, declining by 70% within the short time it takes for Tb to fall from 37oC to 30oC3. Conversely, reversal of this suppression during arousal occurs only gradually, and even after Tb rises from 5oC to 30oC, respiration is only ca. 50% of values from fully aroused, euthermic animals4. This “fast in, slow out” pattern suggests that acute and temperature-sensitive mechanisms are responsible for the reversible suppression of mitochondrial metabolism, and our recent research seeks such mechanisms. Mitochondrial membranes are remodeled during entrance and arousal, with changes in both phospholipid classes and fatty-acyl chain saturation. These membrane changes, however, are transient and do not correlate with mitochondrial respiration5. In torpor, both intact mitochondria and succinate dehydrogenase (SDH) have higher apparent affinity for succinate, but SDH is inhibited by ca. 25%2. The inhibition of SDH is probably caused by oxaloacetate (OAA), a Krebs cycle intermediate, and the apparent affinity of SDH for OAA increases in torpor. Reversal of OAA inhibition (by preincubation with isocitrate) restores SDH activity, but it does not fully “rescue” state 3 respiration to euthermic levels4. Our recent phosphoproteomic analysis revealed seasonal (i.e. summer vs. winter) differences in the phosphorylation state of several mitochondrial proteins, but no differences within winter (i.e. torpid vs. aroused), when the rapid and reversible suppression of oxidative phosphorylation is most evident. Future experiments will examine potential inhibition of oxidative phosphorylation complexes by measuring their redox state in intact mitochondria using rapid-scanning optical spectrometry. We will also examine changes in mitochondrial protein acetylation state using immunoblot analysis, and inhibition of substrate transport, especially by the dicarboxylate carrier which is responsible for succinate transport.