Electrophysiological studies using the single neuron recording technique have led to the hypothesis that nociceptive neurons in the medullary dorsal horn (MDH) and the first segment of the spinal cord (C1) encode the stimulus intensity of noxious stimuli applied to the tooth pulp. The present study utilized the Fos protein technique in combination with electrical and chemical stimulation of the tooth pulp to test this hypothesis. Upper canine tooth-pulp stimulation with intensities just above the threshold stimulus intensity for evoking the jaw-opening reflex (JOR) did not produce a clear expression of Fos protein-like immunoreactive (LI) cells in the MDH and C1 of cats. Fos protein-LI cells were mainly found in the superficial laminae (laminae I–II) of the MDH and C1 after tooth-pulp stimulation of 200% of the JOR threshold intensity. When higher intensities (400–600% of the JOR threshold intensity) or mustard oil were applied, Fos protein-LI cells were also found in laminae III–IV as well as in laminae I–II. The number of Fos protein-LI cells significantly increased when 600% of the JOR threshold intensity or mustard oil was applied. Furthermore, the rostro-caudal distribution of Fos protein-LI cells was greater following increases in stimulus intensities and the greatest after mustard oil application. These data suggest that the change in number and spatial arrangement of nociceptive neurons in the MDH and C1 reflect changes in the encoding of the stimulus intensity applied to the tooth pulp.