The ultrastructural distribution of glycine-like immunoreactivity in laminae I-III of rat spinal dorsal horn was examined by using pre-embedding immunocytochemistry. Immunoreactive axons, dendrites and cell bodies were observed in all three laminae, but were most common in lamina III. The axons were presynaptic at axodendritic and axosomatic synapses, but also at axo-axonic synapses in laminae II and III, where the postsynaptic boutons frequently resembled the terminals of myelinated primary afferents. Some vesicle-containing dendrites in lamina II also showed glycine-like immunoreactivity. Immunoreactive dendrites in laminae II and III were postsynaptic to the central axons of type II, but not type I glomeruli, which suggests that glycinergic neurons receive a major monosynaptic input from myelinated primary afferents. These results support the suggestion that GABA and glycine co-exist in some neurons in laminae I-III of rat dorsal horn, and confirm that glycine is involved in somatosensory processing involving low threshold myelinated cutaneous primary afferents.