Glycolysis from [6-3 H]glucose and gluconeogenesis from [U-14 C]glycerol were examined in isolated hepatocytes from fasted rats. A 5 mMbolus of glycerol inhibited phosphory-lationof 40 mMglucoseby50%andglycolysis bymore than 60%, and caused cellular ATP depletion and glycerol 3-phosphate accumulation. Gluconeogenesis from 5 mM glycerol was una ected by the presence of 40 mMglucose.