After rats were chronically exercised by endurance swimming, an augmentation of succinic dehydrogenase (SDH) activity in the diaphragm and the EDL muscle was observed. In addition, endurance training induced a decrease in the percentage of myosin heavy chain (MHC) IIb in the diaphragm, and a decrease in the percentage of HCIIb concomitant with an increase in the percentage of HCIId in the EDL muscle. These results indicate that endurance training induced not only an increase of the oxidative capacity but also an alteration in the expression of an MHC phenotype in the diaphragm and skeletal muscle.