In healthy man, conditions that change muscle O2 delivery affect the achievable maximum rate of O2 uptake (,max) as well as the metabolic (e.g. lactate threshold, LT) and gas exchange (e.g. gas exchange threshold, The) responses to incremental exercise. Inclined (I) compared to level (L) running increases locomotory muscle EMG at a given speed in the horse, indicative of elevated metabolic demand. To our knowledge, the effect of treadmill incline on ,max, LT and The has not been addressed in the exercising quadruped. We used blood sampling and breath-by-breath expired gas analysis to test the hypothesis that I (10% gradient) would increase ,max and the rate of O2 uptake () at LT and Tge in six Thoroughbred horses during incremental running to volitional fatigue. ,max was significantly higher for I (I, 77.8 ± 4.1; L, 65.5 ± 5.3 l min−1; P<0.05), but peak plasma lactate concentration was not (I, 28.0 ± 3.7; L, 25.9 ± 3.0 mM). Arterial PCO2 increased to 62.1 ± 3.3 and 57.9 ± 2.7 Torr (I vs. L; P<0.05), yet despite this relative hypoventilation, a distinct The was present. This The occurred at a significantly different absolute (I, 49.6 ± 3.2; L, 42.4 ± 3.2 l min−1; P<0.05), but nearly identical relative (I, 63.6 ± 1.2; L, 63.9 ± 1.6% ,max) in I and L. Similarly, LT occurred at a significantly greater absolute (I, 37.3 ± 2.8; L, 26.9 ± 2.1 l min−1), but a relative that was not different (I, 47.9 ± 2.1; L, 43.9 ± 4.5% ,max). In addition, Tge occurred at a significantly higher (P ≤ 0.05) absolute and relative than LT for both I and L tests. In conclusion, ,max is higher during inclined than level running and both LT and Tge in the horse occur at a similar percentage of ,max irrespective of the absolute level of ,max. In contrast to humans, LT is a poor analogue of The in the horse.