Abstract The interaction between both superior colliculi through the collicular commissure was studied in cats. Stimulation of the optic tract and superior colliculus elicited a positive potential on the contralateral superior colliculus, recorded between 1.5- and 4.5- or 5.0-mm depth. This intertectal potential disappeared after commissure sectioning. A reduction in the amplitude of the tectal potential evoked by ipsilateral optic tract stimulation was observed during activation of the contralateral superior colliculus. This suppressive effect was also abolished after commissural section. In contrast, this section produced an increase in the amplitude of the tectal potentials elicited by photic or ipsilateral optic tract stimulation. We conclude that the deeper layers of both superior colliculi are connected through the collicular commissure and that the tectotectal interaction there mediated is inhibitory in nature.