The effect of electrical stimulation of cerebral cortex on neuronal activity in the subnucleus caudalis of the spinal trigeminal nucleus (STNcd) was examined electrophysiologically in the cat. The nociceptive neuronal firing in the STNcd was suppressed by electrical stimulation of contralateral cerebral cortex (primary motor : Msl or primary somatosensory : Sml cortical area.) The high amplitude spontaneous and continuous neuronal hyperactivity (deafferentation hyperactivity : DH), which was provoked in the left STNcd after complete ablation of left Gasserian ganglion, was conspicuously suppressed by the electrical stimulation of the cerebral cortex (Msl stimulation : 5/36 ; Sml stimulation : 12/36). Almost all neurons (nociceptive and DH neuron), the neuronal activity of which was suppressed by electrical stimulation of cerebral cortex, were located in the dorsal and ventral reticular subnuclei of STNcd. On the other hand, the injection of wheat germ agglutinin labeling horseradish peroxidase into the cerebral cortex (Msl) demonstrated that the efferent fibers passed through the internal capsule (IC) and projected directly to the dorsal and ventral reticular subnuclei of the STNcd of the contralateral side. This study suggested that the neuronal firing in STNcd (nociceptive neuronal firing and DH) was suppressed by stimulation of the inhibitory efferent fibers from the cerebral cortex to STNcd by electrical stimulation of IC and cerebral cortex.