Studies were undertaken to determine the nature of the cause(s) for the reported decrease in the measurable hexokinase II activity in the skeletal muscle of the streptozotocin-diabetic rat. An ELISA procedure was developed to determine the amount of hexokinase II protein in the skeletal muscle extracts, and immunoprecipitation was utilized to determine the hexokinase II activity. Both hexokinase II activity and the amount of hexokinase II protein decreased in the diabetic rat. Both increased toward normal treatment with insulin. The specific activity of the hexokinase II was found to be the same in the skeletal muscle extracts from normal, diabetic, and diabetic insulin-treated rats;The rate of synthesis of hexokinase II in the skeletal muscle was determined the rate of incorporation of ('3)H leucine into hexokinase II. This rate was compared to that of the total cytosolic proteins to determine if the rate of hexokinase II synthesis was specifically affected by insulin. This relative rate of synthesis of hexokinase II was found to be approximately two times greater in the normal as compared to the diabetic rat. The administration of insulin to the diabetic animal was found to increase the relative rate of hexokinase synthesis to approximately normal levels;The apparent rate constants of degradation were determined using a double-isotope technique and from these constants it was possible to calculate the apparent half-lives. The apparent half-life of hexokinase II obtained for the normal, diabetic, and diabetic insulin-treated animals was compared to the half-lives of the average of the total cytosolic proteins for each respective state. This was done to determine if the degradation of hexokinase II was specifically affected by insulin or the diabetic state. This corrected apparent half-life was found to be approximately 2.3 times greater in the normal as compared to the diabetic rat and approximately 2 times greater in the insulin-treated as compared to the diabetic;This study suggests that the decrease in measurable hexokinase II activity in the diabetic skeletal muscle is due to a decrease in the synthesis and an increase in the degradation of the enzyme. The results also suggest that insulin may play a role in increasing the synthesis of the enzyme, but does not alter the specific activity of the enzyme.