In contrast to mammals, lampreys are capable of recovering apparently normal locomotion after complete spinal cord transection, and the spinal axons regenerate selectively in their correct paths. Descending serotonergic projections to the spinal cord play a role in the modulation of locomotion at spinal levels in both mammals and lampreys. In this study, we used combined immunofluorescence and tract-tracing techniques to show that in the sea lamprey, serotonergic descending neurons of the caudal rhombencephalon (vagal nucleus) regenerate their axons across the lesion site after complete spinal cord transection. The spinal cord of mature larval sea lampreys was transected at the level of the fifth gill, then after a recovery period of 5 months, the spinal cord was exposed again, 1 mm caudal to the injury site, and the tracer Neurobiotin(™) was applied. Double-labeled cells were observed in the caudal portion of the serotonin-immunoreactive vagal nucleus of the caudal rhombencephalon. In order to investigate whether the reinnervation was due to sprouting from axons above the injury site or to regeneration of axotomized axons, the experiments were performed again, but the tracer Fluoro-Gold(™) was applied at the time of transection. Triple-labeled cells were observed in the vagal nucleus, indicating that at least part of the reinnervation corresponds to true regeneration. This study provides a new and interesting model for investigating the intrinsic molecular mechanisms involved in regeneration of the serotonergic descending axons in vertebrates. Use of this model may provide valuable information for proposing new therapies for patients with spinal cord injury.