The aim of this study was to investigate the effect of high-intensity training on lactate transporters (monocarboxylate transporters (MCTs)) in Thoroughbred horses. Five Thoroughbreds (three males and two geldings; 3–10 years old; 478 ± 13 kg) ran at 100% maximal oxygen uptake (VO2max) for 1 min (3 days per week, 3 weeks) or 2 min (2 days per week, 4 weeks) on a treadmill for 7 weeks. VO2max and ability to sustain an incremental all-out test of maximal running were measured, and muscle samples were obtained from the gluteus medius muscle at rest before and after the training. The performance in the all-out test and VO2max were increased after the training. The protein content of MCT1 and MCT4 in the gluteus medius muscles was not significantly altered. However, MCT1 showed a relationship with the duration of maximal exercise after the training. The changes in MCT4 showed a relationship with the increase in exercise duration by the training. These results suggest that in Thoroughbreds, although the overall changes in MCT protein after endurance training are small, MCT1 and MCT4 can play important roles in the capacity to sustain maximal exercise.