We examined the effects of short-term (5 weeks) and long-term (12 weeks) physical training on actual and total activities, protein content and mRNA abundance of branched-chain 2-oxo acid dehydrogenase complex in rat skeletal muscle. The actual and total activities were significantly increased ∼ 60% and ∼ 40%, respectively, by long term training. No effects of short-term training on activities were observed. The increase in the total activity corresponded to increased protein content of the E1α and E2 components of the complex. On the other hand, mRNA abundance for E1α and E2 were not affected by the training, but that for E1β was slightly, but significantly increased by both short-term and long-term trainings. These divergent alterations of the message levels for the subunits of the complex suggest that posttranslational regulatory mechanisms determine the amount of the complex in skeletal muscle. Since the complex is located in the mitochondrial matrix space, mitochondrial biogenesis in response to the training was examined by determining the content of mitochondrial DNA in the muscle. The mitochondrial DNA was proportionally increased with the total activity as well as the protein content of the complex, suggesting that expression of branched-chain 2-oxo acid dehydrogenase complex in skeletal muscle in response to physical training is associated with mitochondrial biogenesis. © 1995.