The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the nature of the responsiveness of single neurons in and around the PPN following auditory stimulation. Somatosensory responsiveness also was tested in some cells. These results demonstrate a) the presence of a significant proportion of PPN neurons that respond to auditory click stimuli; b) two populations of neurons showing either low threshold/short latency/low habituation or high threshold/longer latency/high habituation; c) the responses of longer latency neurons precede the onset and peak of depth- and vertex-recorded middle-latency auditory-evoked potentials; d) thresholds of longer latency neurons similar to the threshold for wave A in the intact cat, the P13 potential in the intact rat, or the startle reflex; and e) convergent somatosensory and auditory responses at a similar latency in a number of PPN neurons. These findings suggest that neurons in and around the PPN may participate in auditory and somatosensory information processing related to arousal, and may contribute to the manifestation of the P1 auditory middle-latency evoked potential.