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Glucose utilization and production by the isolated perfused rat liver have been examined under a variety of conditions. Rat livers perfused with 20 m m lactate and increasing glucose concentrations from 3 to 55 m m come to a point of equilibrium at which glucose production equals glucose utilization. This occurs with an initial glucose concentration of about 12 m m in the normal fasted rat and 20 m m in the alloxan diabetic. An increase in glucose utilization was observed in livers from diabetic rats with initial perfusate glucose concentrations greater than 30 m m . Such a response is not consistent with the properties of any of the known enzyme systems that convert glucose to glucose-6-phosphate. Further, it has been observed that in prolonged fasting glucose utilization is not depressed to the extent that glucokinase activity is decreased. Although glucose utilization by the normal rat liver would appear to be regulated by glucokinase, alterations observed in alloxan diabetes and prolonged fasting are not consistent with phosphorylation by this enzyme.