Morphine treatment exacerbated locomotor impairment after fracture/pin (FX). Locomotor ability was assessed using the Basso mouse scale (BMS) at 2 and 4 weeks after fracture. (A and B) BMS scores and subscores of both sham and fracture pinning mice treated with and without morphine at 2 weeks after FX. The BMS scores and subscores were significantly lower in the morphine-treated and untreated FX mice compared with morphine-treated and untreated sham mice, respectively. Furthermore, 7-day exposure to morphine in FX mice caused greater locomotor impairment than observed in vehicle-treated FX mice. Morphine treatment had no effect on locomotor function of the sham mice. (C and D) The BMS scores and subscores for mice at 4 weeks after FX were not significantly different when compared with FX mice at 2 weeks after FX, regardless of treatment. However, at 4 weeks after FX, there was no longer a significant difference in the extent of functional recovery between morphine-treated and vehicle-treated mice. Data are expressed as mean values ± SD. **P < 0.01, ***P < 0.001 FX + Vehicle (N = 9) or FX + Morphine (N = 10) versus Sham + Vehicle (N = 10), #P < 0.05 FX + Morphine (N = 10) versus FX + Vehicle (N = 9).