Serotonin selectively depresses transmission of nociceptive information through the spinal dorsal horn but the mechanisms of this depression are poorly understood. In this study we report that serotonin-containing axons form basket-like clusters which are intimately woven around cell bodies and proximal dendrites of a subpopulation (≈ 50%) of laminae III/IV neurons which possess the neurokinin-1 receptor. Statistical analysis confirms that cells belonging to this subpopulation have significantly higher numbers of serotoninergic contacts on proximal dendrites when compared with the population of neurokinin-1 cells that are not associated with clusters (mean ± SD = 13 ± 5.8 and 5 ± 2.9, respectively). Neurokinin-1 cells in laminae III/IV project to regions of the brain which are involved in nociceptive processing and are likely to be activated predominantly by nociceptive input. The concentration of serotoninergic axons around proximal regions of some of these cells indicates that serotonin may have a powerful influence on transmission through this pathway. This type of arrangement could be a morphological correlate for at least part of the selective antinociceptive actions of serotonin.