1. Single-unit recording has been performed from the superficial layers of the superior colliculus of regionally anesthetized cats. The response of the collicular units to a luminous bar moving in its receptive field has been analyzed as a function of the position of rotation of the animal about its longitudinal axis. 2. Forty-eight percent of collicular directional units altered their visual responses with an approximately linear function of the body tilt. 3. Twenty-five percent of collicular nondirectional units changed their visual responses following body tilt. They showed no clear relation between amplitude of response and angle of tilt. 4. No variation in the preferred direction of stimulus movement of directionally selective collicular units was observed following body tilt. 5. Analysis of collicular responses after chronic section of the eighth nerves in five cats strongly indicated that the vestibular system plays a major role in the modification of visual responses at collicular level.