Isolated livers from fed and fasted rats were perfused for 30 min with recirculating blood-buffer medium containing no added substrate and then switched to a flow-through perfusion using the same medium for an additional 5, 10 and 30 min. Continuous infusion of fructose for the final 5, 10 or 30 min resulted in activation of glycogen phosphorylase, an increase in the activity of protein kinase, elevated levels of tissue adenosine 3', 5'-monophosphate (cyclic AMP), and no consistent effect on glycogen synthase. Infusion of glucose under the same conditions resulted in activation of glycogen synthase, inactivation of glycogen phosphorylase, no change in protein kinase, and no consistent change in tissue cyclic AMP. These results demonstrate that while glucose promotes hepatic glycogen synthesis, fructose promotes activation of the enzymatic cascade responsible for glycogen breakdown.