gamma-Aminobutyric acid (GABA)ergic and glycinergic neurons were examined light- and electron-microscopically in laminae I and II of the medullary dorsal horn (MDH, i.e. spinal trigeminal nucleus caudalis in the rat). The majority of GABA- and glycine (Gly)-immunoreactive (-ir) neurons showed both GABA- and Gly-immunoreactivities (-IRs). Noxious stimulation (subcutaneous injection of formalin into perioral regions) induced Fos-IR in some of GABA- and Gly-ir neurons. GABA- and Gly-ir neuronal profiles were postsynaptic to substance P-ir axon terminals. These results suggest that nociceptive information being carried by primary afferent SP-fibers may be relayed directly to GABAergic and glycinergic neurons in laminae I and II of the MDH.