Our previous study reported that the afferents to the spinal trigeminal nucleus from the masseter muscle were predominantly of A-delta range fibers having fast conducting-low threshold and slow conducting-high threshold. In the present study, further experiment was undertaken to obtain more definite information on the central transmission of muscle pain. When the electrical stimulation was delivered to the masseter muscle of rat, the evoked potentials with a shorter latency (the S-response) or longer latency (the L-response) were recorded from the caudal part of the spinal trigeminal nucleus. In both the S- and the L-responses, the relationship between the amplitudes of the responses and the stimulus intensity almost followed the power function with the exponent of about 3.4 and 4.0, respectively. The stimulus intensity range of the S-response, in which the power function was formed, was wider than that of the L-response. The most of the S-response were recorded from the trigeminal subnucleus caudalis and adjacent reticular formation, whereas the recording sites of the L-response were located mainly in the trigeminal subnucleus caudalis. These results suggest that the trigeminal subnucleus caudalis and adjacent reticular formation are an important portion of the afferent projection areas and of the modulator system of pain sensation from the masseter muscle.