The exercising Thoroughbred horse (TB) is capable of exceptional cardiopulmonary performance. However, because the ventilatory equivalent for O₂ (E/O₂) does not increase above the gas exchange threshold (Tge), hypercapnia and hypoxemia accompany intense exercise in the TB compared with humans, in whom E/O₂ increases during supra-Tge work, which both removes the CO₂ produced by the HCO buffering of lactic acid and prevents arterial partial pressure of CO₂ (Pa_CO2) from rising. We used breath-by-breath techniques to analyze the relationship between CO₂ output (CO₂) and O₂ [V-slope lactate threshold (LT) estimation] during an incremental test to fatigue (7 to ~15 m/s; 1 m · s¹ · min¹) in six TB. Peak blood lactate increased to 29.2 ± 1.9 mM/l. However, as neither E/O₂ nor E/CO₂ increased, Pa_CO2 increased to 56.6 ± 2.3 Torr at peak O₂ (O_2 max). Despite the presence of a relative hypoventilation (i.e., no increase in E/O₂ or E/CO₂), a distinct Tge was evidenced at 62.6 ± 2.7% O_2 max. Tge occurred at a significantly higher (P < 0.05) percentage of O_2 max than the lactate (45.1 ± 5.0%) or pH (47.4 ± 6.6%) but not the bicarbonate (65.3 ± 6.6%) threshold. In addition, Pa_CO2 was elevated significantly only at a workload > Tge. Thus, in marked contrast to healthy humans, pronounced V-slope (CO₂/O₂) behavior occurs in TB concomitant with elevated Pa_CO2 and without evidence of a ventilatory threshold.