Facial motor responses to microstimulation of different zones of the superior colliculi have been investigated in the albino mice craniotomized under thiopental anaesthesia. Local responses of the mystacial vibrissae, upper lip and eyelids were initiated by microstimulation of the rostral parts of the inner layers of the colliculus superior (high-frequency volleys of 5-7 pulses with a current limit of 35 microA). Sequential changes in the pattern of facial responses were observed within microelectrode traces indicating vertical orientation of facial motor representations in the superior colliculus. Some differences in the localization and pattern of facial responses in the right and left superior colliculi were revealed: 1) vibrissae and lip representations in the right superior colliculus occupy more extensive zone (vertical distribution from 300 to 2,300 microns) as compared to those in the left one (700-2,000 microns); 2) microstimulations of the right superior colliculus produce both uni- and bilateral vibrissal motor responses, whereas stimulation of the left superior colliculus evokes only unilateral responses. The duration of the latent period of the vibrissal and lip motor responses to stimulation of the right superior colliculus varied from 10 to 26 ms (16.1 +/- 2.4 ms; n = 199), to stimulation of the left one-from 10 to 18 ms (mean 14.9 +/- 1.8 ms; n = 55). It is suggested that polysynaptic motor responses to microstimulation of the superior colliculi are realized via the reticular and other premotor nuclei of the brain stem which have direct inputs from the superior colliculus and direct projections to the facial motor nucleus.