The branched-chain alpha-ketoacid dehydrogenase (BCKDH) complex is the rate-limiting enzyme in the catabolism of branched-chain amino acids. In the present study, we examined the effects of exercise training on the activity and enzyme expression of the hepatic BCKDH complex in diabetic rats. The rats were prepared by intravenous injections of streptozotocin (50 mg/kg BW), and exercise training was accomplished by treadmill running for 45 min/d for 4 wk. The total and actual activities of hepatic BCKDH complex were significantly increased to approximately 160% by 4 wk of diabetes. On the other hand, diabetic rats in the trained group had the same level of activities as those in the normal rats, indicating that exercise training inhibited the diabetes-induced increase in the enzyme activities. The activity state (% active form) of the enzyme complex was about 100% in all groups and was not affected by diabetes or training. The protein amounts of the enzyme subunits (E1alpha and E2) and the abundance of mRNA for the E2 subunit, but not for the other subunits, in the liver had the same trend as the activities. These results suggest that the capacity for branched-chain amino acid catabolism in streptozotocin-induced diabetic rats is reduced by exercise training and that this modification is associated with the suppression of diabetes-induced BCKDH complex expression in the liver.