Thoroughbred (TB) and draft horses (DH) have long been selected for tasks of very different intensities and force-speed relationships. To study their adaptations, we measured O₂ consumption and related variables in three TB and four DH during progressive exercise tests on a level treadmill. The horses exerted a draft force of 0, 5, 10, 15, or 20% of their body weight at speeds that increased by 2 m/s every 3 min until they could not maintain that speed. We found that TB could exert the same draft forces as DH and, at each force, TB achieved about twice the speed, twice the external power, and twice the O₂ consumption as DH; thus the two breeds had the same gross efficiencies. We also found maximal O₂ consumption of TB to be about twice that of DH (134 vs. 72 ml · kg¹ · min¹, respectively), suggesting adaptations to high-intensity exercise. Peak efficiency was reached at lower speeds in DH than in TB, suggesting adaptations to high-force, low-speed exercise. These differences between TB and DH in force-speed and aerobic capacities and in speed for peak efficiency likely reflect different contraction velocities in locomotor muscles.