Descending serotoninergic systems in the spinal cord may be involved in antinociception. In this study 6 rat spinal cords were prepared for serotonin immunostaining and marginal zones (lamina I of Rexed) from the dorsal horn were examined with electron microscopy. Stained terminals were often seen to be closely associated with the somata of lamina I neurones or other unstained axon terminals, but seldom formed conventional synaptic junctions. It is concluded that serotoninergic systems in this region may influence the activity of these associated structures in a diffuse non-synaptic manner.