Hepatic microsomal glucose-6-phosphatase activity levels and the hepatic output of glucose are increased in diabetes. We have used protein chemistry and immunological techniques to determine the mechanism by which the activity levels of the glucose-6-phosphatase system are increased in streptozotocin-induced diabetic rats. In the streptozotocin-induced diabetic rats, the activity of the glucose-6-phosphatase enzyme increased four-fold without appreciably altering the transport capacity of the glucose-6-phosphatase system. The solubilized diabetic rat liver glucose-6-phosphatase enzyme appeared to be very similar to the solubilized enzyme from control rat liver microsomes. They exhibit the same Km, are labile at 30 degrees C, are stabilized by sodium fluoride and they migrate to the same position during density gradient centrifugation. Immunological studies demonstrated that a greater amount of hepatic microsomal glucose-6-phosphatase enzyme protein is present in diabetic rats than in control rats. Thus, we have determined for the first time that increased levels of the glucose-6-phosphatase protein are present in streptozotocin-induced diabetes. The significance of this finding in relation to the regulation of the hepatic microsomal glucose-6-phosphatase system is discussed.