Sleep-induced changes in the trigeminal neuron responses to electrical stimulation of the cat tooth pulp were studied. Two parameters were adopted: One was the evoked spike number at two times the threshold intensity (2 X T response magnitude), which would reveal the level shifting of the neuronal response by the sleep-regulatory system. Another was the rate of change in the response intensity when the stimulus was raised to a level of 0.7 time the arousal threshold during light slow wave sleep (sensitivity gradient), which would reflect the influences of the pain-modulatory system driven by strong noxious inputs. It was found that during sleep the two indexes tended to show a correlated change; the neurons which came to have a greater 2 X T response magnitude tended to have a smaller sensitivity gradient than during wakefulness, and vice versa. It was suggested that two contrasting populations of tooth pulp neurons might be differentiated, and that the sleep-regulatory system and the pain-modulatory system would have differential but correlated controls over these two kinds of neurons.