To evaluate the clinical efficacy of medetomidine constant rate infusion (CRI) in sevoflurane-anesthetized Thoroughbred racehorses undergoing arthroscopic surgery. Sevoflurane requirement and cardiopulmonary function were compared between horses maintained with sevoflurane alone (group S; n = 25) and those maintained with sevoflurane in combination with medetomidine CRI at a rate of 0.05 μg/kg/min (group SM; n = 25). Recovery quality was also compared between the two groups. A single IV bolus of 1.0 μg/kg medetomidine was administered to the horses in group S at the end of anesthesia, whereas no additional sedatives were administered in group SM. Mean end-tidal sevoflurane concentrations were significantly lower in group SM (2.5 ± 0.1%) than in group S (2.8 ± 0.1%). Mean arterial blood pressure (MAP) values were significantly higher in group SM than in group S. On the other hand, mean dobutamine infusion rate required for maintaining MAP within the target values (60–80 mm Hg) was significantly lower in group SM than in group S. The number of attempt to stand was significantly fewer, and recovery score was significantly better in group SM compared with group S. In conclusion, medetomidine CRI reduced sevoflurane requirement by approximately 10% with good maintenance of cardiopulmonary function and better recovery quality.