The clinical implications of ventilatory alterations induced by left recurrent laryngeal neuropathy (LRLN) are reduced performance due to increased metabolic requirements for breathing and a reduction in peak aerobic power (V̇O2peak). To test this hypothesis, the metabolic cost of locomotion (MCL) and peak aerobic power (V̇O2peak) were measured in 4 non racing crossbred horses (low V̇O2peak) with reversibly induced LRLN using an incremental treadmill exercise test. Reversible LRLN was achieved with regional anaesthesia. These results were compared to previous measurements in 4 Thoroughbred racehorses (high V̇O2peak, Ehrlich et al. 1993). No significant difference in MCL (based on V̇O2 and venous lactate) was found in either group of horses at exercise intensities where V̇O2 < V̇O2peak (<9 m/s). Peak aerobic power decreased by 15.3% (140.0 ± 3.2 vs. 165.3 ± 3.4 ml/kg/min ± s.e., in the Thoroughbred racehorses (high V̇O2peak) with induced LRLN but not in the non racing horses with lower V̇O2peak (135.0 ± 4.5 ml/kg/min ± s.e. The results of these 2 studies indicate that alterations in ventilation due to LRLN cause a significant reduction in V̇O2peak only in horses with high V̇O2peak. At exercise intensities where V̇O2 < V̇O2peak, MCL was not increased in either group of horses despite ventilatory alterations induced by LRLN.