The hibernation season in the arctic ground squirrel (Citellus undulatus) is broken into 8- to 18- day cycles by short homeothermal periods during which the carboydrate reserves depleted during hibernation are replenished. This study follows a number of metabolities in tissues and body fluids to assess the sources for reconstitution of the glucose reserves: lactate, urea, ammonia, free fatty acid, glycerol, triglyceride, and glucose in plasma; glycogen in liver and muscle; and urea and ammonia nitrogen in urine. Fat is the major energy source during both homeothermal and heterothermal periods, the contribution from glucose being limited to glycolysis. Reconstitution of glycogen is accomplished prior to reentry through maximal use of substrates from all sources including glycolysis, fat, and protein metabolism. Of the new gluconeogenic substrate, one-fourth is supplied from protein and three-fourths from fat.