ABSTRACT. To evaluate the effects of IV fentanyl administration on the end-tidal sevoflurane concentration (ET SEVO ) in thoroughbred racehorses, the ET SEVO required for internal fixation of longitudinal fractures was compared between horses anesthetized with sevofluranefentanyl (Group SF; n=9) and those anesthetized with sevoflurane alone (Group S; n=9). The loading dose of fentanyl (5.0 g/kg) was administered over 15 min followed by a maintenance dose of fentanyl (0.1 g/kg/min) throughout the operation in Group SF. The mean ET SEVO during the operation in Group SF (2.6 0.2%) was significantly lower than in Group S (3.0 0.3%). The plasma fentanyl concentrations (6.12 0.88 to 7.78 1.12 ng/ml) in 7 out of 9 horses in Group SF were stable and did not change significantly throughout the operation. The mean dobutamine infusion rate required for maintaining a mean arterial blood pressure between 60 and 80 mmHg during the operation in Group SF (0.56 0.30 g/kg/min) was significantly lower than in Group S (0.90 0.16 g/kg/min). The qualities of the recoveries were clinically acceptable, and serious complications were not observed in either group. In conclusion, continuous IV fentanyl administration reduced the sevoflurane requirement by 13% in thoroughbred racehorses undergoing orthopedic surgery; however, fentanyl was considered to be less effective in horses compared with other species.