Rat livers perfused with 20 mM sodium lactate and increasing glucose concentrations from 0 to 1000 mg/100 ml come to a point of equilibrium at which glucose production equals glucose utilization. This occurs with an initial glucose concentration of about 200 mg/100 ml in the normal fasted rat, 60 mg/100 ml for the adrenalectomized rat and 340 mg/100 ml for the alloxan diabetic rat. An increase in glucose utilization was observed in diabetic livers between 300-600 mg 100 ml. This is thought to be due to the action of pyrophosphate phosphotransferase. There was a plateau in glucose utilization in diabetic livers between 600-1000 mg/100 ml initial glucose concentration in the medium, and it is thought this represents an equilibrium between glucose and glucose-6-phosphate.