For many animals a reduction in metabolism represents an important adaptation to food deprivation. Given that the digestive organs are considered energetically expensive, considerable emphasis has been placed on the ability to down- regulate the size and function of these organs during fasting. Here we measured whole animal metabolism by open respirometry and characterized the glucose metabolism of various organs during a three-days fasting period in rats that had been raised on either ad libitum (AL) or restrictively (lean) diets. By three days fasting body mass decreased around 11% in both groups, basal metabolic rate was reduced by 12% and the depletion of glycogen levels in muscle and liver together with a reduction in respiratory exchange ratio to 0.67 showed that the rats were predominantly combusting lipid. Metabolism of the visceral organs was determined by the uptake of fludeoxyglucose (¹⁸F-FDG); a radioactive tracer that resembles glucose, but is metabolically trapped in the cells. The ¹⁸F-FDG accumulation, therefore, is considered to be proportional to glucose metabolism. During the three days fasting period the gastrointestinal organs percentage of body mass was reduced with 18% (AL) and 13% (lean). Irrespective of the digestive state the digestive organs are responsible for 11.4 ± 3 % (AL) and 21.4 ± 5 % (lean) of glucose metabolism. Our study shows that although the mass and morphology of the gastrointestinal organs is reduced during the three days of food deprivation, there is very little down- regulation of the metabolism of these digestive organs in fasting rats.