The masseter inhibitory period and sensations evoked by electrical tooth pulp stimulation were assessed in 30 human subjects. Five intensities of electrical stimuli, producing sensations varying from below sensory detection threshold to suprathreshold pain, were applied to upper central incisors. At each stimulus intensity a train of 30, 1-msec, cathodal pulses with an interpulse interval of 2 sec was applied. The averaged masseter activity evoked by the 30 pulses at a fixed stimulus intensity was compared to the quality of the sensation elicited. The threshold for the masseter inhibitory period coincided approximately with an individual's detection threshold for the tooth pulp stimulation. Three configurations of masseter inhibitory periods (single, double and merged) were produced by different stimulus intensities. However, no particular configuration was associated unequivocally with pain sensation. Increases in stimulus intensity evoked changes both in the configuration of the masseter inhibitory period and in the quality of the sensation produced. Chi square analyses showed significant, but progressively weaker, associations between: (1) masseter inhibitory period configuration and stimulus intensity; (2) quality of sensation and stimulus intensity; and (3) quality of sensation and masseter inhibitory period configuration. The weakness of the association between the quality of sensation and masseter inhibitory period configuration also was demonstrated in a double-blind study of the effects of a narcotic analgesic, fentanyl. Although the strengths of non-pain and pain sensations were reduced significantly after fentanyl, there were no changes in the masseter inhibitory periods.