W. ROBERT COOK, BVSc, FRCVSThe effect of left recurrent laryngeal neuropathy (LRLN) on the metabolic cost of locomotion (MCL) and peak aerobic power (V̇O2peak) was evaluated in four trained Thoroughbred racehorses. Oxygen consumption (V̇O2), carbon dioxide production (V̇CO2), venous lactate concentrations (LAC), and heart rate (HR) were measured during a treadmill exercise test (TET). Each horse performed the exercise test four times, alternating between normal upper airway function and reversibly induced LRLN. Subcutaneous infusion of 2% mepivicaine, a local anesthetic, into the region where the left recurrent laryngeal nerve passes caudal to the cricoid cartilage was used to induce LRLN. The induction of LRLN did not alter the relationship between V̇O2and treadmill speed at exercise intensities where V̇O2was less than V̇O2peak (<9 m/sec). However, a 15.3% reduction in V̇O2peak (Normal = 165.3 ± 3.4, LRLN = 140.0 ± 3.2 mL/kg/min ± SE,P<.001) occurred at higher treadmill speeds in horses with induced LRLN. A significant group (Normal v LRLN) by treadmill speed effect was found for LAC and R only at treadmill speeds where V̇O2=V̇O2peak. Peak lactate (LACpeak) did not change after the induction of LRLN. The relationship between HR and treadmill speed increased in horses with induced LRLN at exercise intensities where V̇O2