Free fatty acid (FFA) levels were determined in warm- and cold-acclimated white rats, mice, and red squirrels (Tamiasciurus hudsonicus). FFA concentrations of the genetically cold-adapted species (red squirrel) were highest and reflect its ability to maintain high metabolic rates reported by others. In the red squirrels, FFA concentrations were elevated after cold acclimation. Acclimation of white rats and mice had no effect on FFA levels. Short-term cold exposure (−35 °C) caused the FFA levels to rise in all three species. Cold-exposed mice were found to be highest in plasma FFA content. The ability of the red squirrels to mobilize FFA during short-term exposure was enhanced by cold acclimation. The response of rats and mice to cold exposure was not changed by acclimation. This would indicate a greater capacity of the red squirrel to respond to acclimating conditions.