Hypoxic training is effective for improving athletic performance in humans. It increases maximal oxygen consumption (V̇O₂max) more than normoxic training in untrained horses. However, the effects of hypoxic training on well-trained horses are unclear. We measured the effects of hypoxic training on V̇O₂max of 5 well-trained horses in which V̇O₂max had not increased over 3 consecutive weeks of supramaximal treadmill training in normoxia which was performed twice a week. The horses trained with hypoxia (15% inspired O₂) twice a week. Cardiorespiratory valuables were analyzed with analysis of variance between before and after 3 weeks of hypoxic training. Mass-specific V̇O₂max increased after 3 weeks of hypoxic training (178 ± 10 vs. 194 ± 12.3 ml O₂ (STPD)/(kg × min), P 2max. Absolute V̇O₂max also increased after hypoxic training (86.6 ± 6.2 vs. 93.6 ± 6.6 l O₂ (STPD)/min, P 2max even though it is not increased by normoxic training in well-trained horses, at least for the durations of time evaluated in this study. Training while breathing hypoxic gas may have the potential to enhance normoxic performance of Thoroughbred horses.