AbstractThe flux through branched‐chain α‐ketoacid dehydrogenase and the activity of the branched‐chain α‐ketoacid dehydrogenase complex were measured in hepatocytes isolated from fed, starved and alloxan diabetic rats. The highest rate of branched‐chain α‐ketoacid oxidation was found in hepatocytes isolated from starved rats, slightly lower in those from fed rats, and significantly lower in diabetic hepatocytes. The amount of the active form of branched‐chain α‐ketoacid dehydrogenase was only slightly diminished in diabetic hepatocytes, whereas the flux through the dehydrogenase was inversely correlated with the rate of endogeneous ketogenesis. The same was observed in hepatocytes isolated from starved rats when branched‐chain α‐ketoacid oxidation was measured in the presence of added oleate. In both cases the diminshed flux through the dehydrogenase, restored by a short preincubation of hepatocytes with insulin, was parallelled by a decrease of fatty acid‐derived ketogenesis. The significance of these findings is discussed in relation to the role of insulin in branched‐chain α‐ketoacid oxid