The speed producing the maximal lactate steady state (maxLASS) is supposed to be the optimal speed to condition for endurance. The maxLASS was defined as the maximal speed at which the blood lactate concentration ([LA]) between the 5th and the 25th min of continuous exercise did not increase by more than 1 mmol/L. According to the aerobic-anaerobic lactate threshold concept determined in humans, maxLASS corresponds to v(4) [speed in a standardized exercise test (SET) shown to produce an [LA] of 4 mmol/L; generalized to v(i) for the speed producing an [LA] of i mmol/L]. Four Thoroughbreds were submitted to a treadmill-based SET to determine their blood lactate-running speed (BLRS) relationship and calculate the individual v(1.5), v(2), v(2.5), v(3), and v(4) values (velocities run under defined conditions inducing 1.5, 2, 2.5, 3, and 4 mmol/L of blood LA). Afterward, horses ran on the treadmill for 40 min at their v(1.5), v(2), and v(2.5) every 3 d. Another 14 horses were submitted to SET in the field to determine their BLRS relationships and to calculate their v(2). The day after the SET, these horses ran once between 15 and 30 min at their v(2). In the horses that ran on the treadmill, maxLASS only occurred when running at their v(1.5). Blood [LA] did not increase by more than 1 mmol/L between the 10th min and the end of exercise for all the horses that ran in the field at their v(2.) These data indicate that maxLASS of horses is not greater than v(2) and therefore less than in running humans.