The key enzyme regulating oxidation of branched-chain keto acids (BCKAs) is BCKA dehydrogenase (BCKAD). We have previously shown that an increase in the activity of this enzyme accounts for the increased oxidation of leucine in the liver of diabetic rats. In the present experiment, we have investigated the mechanisms responsible for this increase in enzyme activity. These studies were performed 96 hours after the withdrawal of insulin therapy in rats made diabetic by an injection of streptozotocin. Diabetes increased the activity state (83% versus 97%, p < .01) as well as the total activity (78 versus 112 nmol/min/mg protein, p < .01) of BCKAD. The increase in the activity state was due to a 60% fall in the BCKAD kinase activity, which was the result of a 50% decrease in its protein mass. A coordinated increase (50%-70%) in protein mass of each BCKAD subunit (E1 alpha, E1 beta, and E2) accounted for the increase in the total activity of BCKAD. We conclude that diabetes increases the hepatic BCKAD activity by increasing its protein mass and also by decreasing that of its associated kinase. These alterations appear to occur posttranscriptionally, since diabetes had no effect on the gene expressions of BCKAD subunits (E1 alpha, E1 beta, and E2) or BCKAD kinase.