The effects of concurrent hy- poxic/endurance training on mitochondrial respiration in permeabil- ized fibers in trained athletes were investigated. Eighteen endurance athletes were divided into two training groups: normoxic (Nor, n = 8) and hypoxic (H, n = 10). Three weeks (W1–W3) of endurance training (5 sessions of 1 h to 1 h and 30 min per week) were completed. All training sessions were performed under normoxic [160 Torr inspired PO2 (PIO )] or hypoxic conditions (=100 Torr PIO2 ,=3,000 m) for Nor and H group, respectively, at the same relative intensity. Before and after the training period, an incremental test to exhaustion in normoxia was performed, muscle biopsy samples were taken from the vastus lateralis, and mitochondrial respiration in permeabilized fibers was measured. Peak power output (PPO) in- creased by 7.2% and 6.6% (P < 0.05) for Nor and H, respectively, whereas maximal O2 uptake (V˙ O2 max) remained unchanged: 58.1 ± 0.8 vs. 61.0 ± 1.2 ml• kg-1 • min-1 and 58.5 ± 0.7 vs. 58.3 ± 0.6 ml• kg-1 • min-1 for Nor and H, respectively, between pretraining (W0) and posttraining (W4). Maximal ADP-stimulated mitochondrial respiration significantly increased for glutamate + malate (6.27 ± 0.37 vs. 8.51 ± 0.33 µmol O2 • min-1 • g dry weight-1) and signifi- cantly decreased for palmitate + malate (3.88 ± 0.23 vs. 2.77 ± 0.08 µmol O2 • min-1 • g dry weight-1) in the H group. In contrast, no significant differences were found for the Nor group. The findings demonstrate that 1) a 3-wk training period increased the PPO at sea level without any changes in V˙ O2 max, and 2) a 3-wk hypoxic exercise training seems to alter the intrinsic properties of mitochondrial function, i.e., substrate preference.