Introduction: Microbiopsies of the M.adductor magnus, which is an important locomotor muscle were taken to study the effects of six weeks of moderate aerobic treadmill training and their reversibility after a training pause of nine weeks in a group of seven Foxhound-Boxer-Ingelheim Labrador crossbreds. Materials and Methods: To study changes of myofibre morphometry standard morphometric methods were applied using light and transmission electron micrographs. Results: The capillary density was higher in trained condition than in untrained condition and after the end of training. In contrast, the capillaries per fiber as well as the capillary?to?fiber ratio, the distance between neighboring capillaries and the capillary supply area showed no significant change. Smaller myofibre diameters were found in muscles of dogs in trained condition when compared to the untrained condition but not when compared to myofibres nine weeks after the end of training. Training led to increases in the volume fraction of mitochondria within myofibres and an increase of the surface density of cristae within the mitochondria. The ratio of the surface density to the volume fraction of the mitochondria increased in response to training. Except for the decrease in myofibre diameter, all of the reported changes were reversible and measurements nine weeks after cessation of training did not differ from measurements before training started. Conclusion: The results of this study emphasize the ability of dogs to rapidly and flexibly adjust locomotor muscles to changing exercise regimes by up-and down regulation of the number and inner structure of the mitochondria while the capillary network remains stable.