Inspiring a hyperoxic gas permits subjects to exercise at higher power outputs while training, but there is controversy as to whether this improves skeletal muscle oxidative capacity, VO₂max and endurance performance to a greater extent than training in normoxia (N). To determine whether the higher power output during H training leads to a greater increase in these parameters, nine recreationally active subjects were randomly assigned in a single blind fashion to train in H (60% O₂) or N for 6 wks (3 sessions/wk of 10 x 4 min at 90% VO₂max). Training HR was maintained during the study by increasing power output. After at least 6 wks of de-training, a second 6 wk training protocol was completed with the other breathing condition. VO₂max and cycle time to exhaustion at 90% of pre-training VO₂max were tested in room air pre and post training. Muscle biopsies were sampled pre and post training for citrate synthase (CS), -hydroxyacyl-CoA dehydrogenase (-HAD) and mitochondrial aspartate aminotransferase (m-AsAT) activity measurements. Training power outputs were 8% higher (17 W) in H vs N. However, both conditions produced similar improvements in VO₂max (11-12%), time to exhaustion (~100%), and CS (H, 30%; N, 32%), -HAD (H, 23%; N, 21%) and m-AsAT (H, 21%; N, 26%) activities. We conclude that the additional training stimulus provided by training in H was not sufficient to produce greater increases in the aerobic capacity of skeletal muscle, and whole body VO₂max and exercise performance as compared to training in N.