Responses to light stimuli of cells in the superior colliculus (SC) were studied in anesthetized and paralyzed rabbits after reversible inactivation of corticocollicular impulses by cooling of the visual cortex. The recording of flash-evoked responses and EEG allowed us to monitor the time course of the inactivation and its recovery. The cell's responses were measured prior to, during, and after cortical blockade. In about half of the collicular cells, disruption of cortical functions resulted in a sharp decline of collicular responsivity. All types of response were affected equally. Hence, the cortex has a global tonic excitatory impact.