An overnight fast of rats leads to decrease in the adenosinetriphosphate (ATP) content of the liver, with corresponding increases in the adenosinediphosphate (ADP) and adenosinemonophosphate (AMP) contents. Glucose administration to the fasted animal does not restore the ATP level to that in the fed animal. Alloxan diabetes leads to a moderate decrease in total adenosine phosphates per gram of liver, with marked decrease in ATP content, and absolute increase of AMP above the highest values found in fasted normal animals. A single injection of insulin causes some increase in the ATP content. Diabetic animals fasted overnight show greater ATP levels in the liver than fed ones. It is postulated that the low levels of ATP found in the livers of the diabetic animals may be the rate limiting factor in synthetic reactions, and that insulin in some way accelerates the rate of formation of glucose-6-phosphate without necessarily restoring the ATP concentration to normal levels.