To determine the role of serotonin (5-HT) in recovery from spinal cord injury, we examined spinal cord 5-HT immunohistologically and assessed locomotor recovery after thoracic (T8) spinal cord hemisection in 68 rats. Forty eight rats had laminectomy and hemisection, while the remaining 20 rats received laminectomy only. All rats were evaluated every other day for 4 weeks, using a 0-14 point scale open field test. Hemisection markedly reduced mean hindlimbs scores from 14 to 1.5 +/- 0.32 and 5.6 +/- 0.31 (mean +/- standard error of mean) in the ipsilateral and contralateral side, respectively. The rats all recovered apparently normal walking by 4 weeks. The 5-HT immunohistological study revealed a marked reduction of 5-HT-containing terminals in the ipsilateral but not the contralateral lumbosacral cord by 1 week after hemisection. By 4 weeks after hemisection, 5-HT immunoreactive fibers and terminals returned to the ipsilateral lumbosacral cord, with many 5-HT fibers crossing over the central canal at thoracic level. We estimated the recovery of 5-HT neural elements in lumbosacral ventral horn by ranking 5-HT staining intensity and counting 5-HT terminals. The return of 5-HT immunoreactivity of the lumbosacral ventral horn correlated with locomotor recovery. Locomotory recovery invariably occurred when the density of 5-HT terminals approached 20% of control values. These results indicate that return of 5-HT fibers and terminals predict the time course and extent of locomotory recovery after thoracic spinal cord hemisection.