Protein levels of mitochondrial transcription factor A (Tfam) and nuclear- and mitochondrial-encoded subunits of respiratory chain complex IV (COX I and COX IV) as well as citrate synthase activity were analysed in muscle biopsy samples of vastus lateralis in six healthy male subjects before and after 4 weeks of one-legged cycle training. One leg was trained with restricted blood flow. The other leg was trained with the same power profile but with non-restricted blood flow. Tfam, COX I and COX IV levels all increased with training, with no differences observed between the legs. The training-induced increase in citrate synthase activity was greater in the leg trained with restricted blood flow. These findings indicate that changed expression of Tfam protein could be one mechanism of exercise-induced mitochondrial biogenesis. The increases of COX I and COX IV indicate a concurrent increase of nuclear- and mitochondrial-encoded subunits of respiratory enzyme complex IV at the protein level in skeletal muscle in response to increased muscle activity. In this study, it was not possible to demonstrate that the greater energy disturbance induced by reduced blood flow further stimulates the expression of mitochondrial proteins, even though it did cause a greater enhancement of citrate synthase activity in concordance with earlier studies.