The reflex responses of single motor units in human masseter muscle to electrical stimulation of the lip were recorded. The subject maintained a content mean level of pre-stimulus excitation of the parent motor neurone by biting in such a way that the unit fired at either 10 or 15 Hz during each trial. When firing at 10 Hz, most units were reflexly inhibited for up to 90 ms by electrical stimuli at intensities that were perceived to be mildly uncomfortable. In many units, the inhibition consisted of 2 phases which were separated from each other by a few spikes occurring about 30 ms after the stimulus. It was occasionally possible to evoke only the latter phase (latency about 40 ms) with stimulus at intensities near the response threshold. In these instances, the inhibitory response became biphasic at higher stimulus intensities with the emergence of a shorter (10-15 ms) component. Still higher intensities caused the 2 phases of inhibition to merge, giving the appearance of a single, prolonged, inhibitory response. When the pre-stimulus firing frequency of the unit was changed from 10 Hz to 15 Hz, the inhibitory responses to the same stimuli were decreased, with the longer-latency component usually surviving beyond the shorter-latency phase. The pattern of reflex responses observed can be explained by a model based on information derived from intracellular recordings in animal experiments.