In the cat, cells of the superior colliculus (SC) and the tectofugal pathways of the visual system are highly selective for the direction of a moving visual stimulus. Deep layer units of SC in addition respond to auditory and somatosensory stimuli, but the proportion of such non-visual cells is usually found to be much lower than that of visual cells. We recorded the responses of 174 cells in the SC to sequentially presented, localized visual and/or auditory stimuli that produced the sensation of apparent motion to human observers. Controls using single LED flashes or tone pips or clicks at very long intervals that did not produce apparent motion were also used. We found both visual and auditory units that responded vigorously to the apparent motion stimuli and showed pronounced directional selectivity. However, in the auditory domain such units were rare and thus did not increase the proportion of auditory responses in SC substantially. Varying the interstimulus interval (ISI) of these stimuli, both visual and auditory, indicated that the mechanism of direction selectivity in these cells was suppression of the response in the 'non-preferred' direction rather than facilitation in the 'preferred' direction. With long ISI's of 200 ms or more, every single stimulus gave a discrete response peak of constant amplitude. For ISI's of 50 ms or less the discrete peaks merged to a continuous response. Maximal firing rate in the preferred direction remained the same as for longer ISI's, but was decreased for movement in the non-preferred directions. Very short ISI's (10 ms) produced little response in any direction.(ABSTRACT TRUNCATED AT 250 WORDS)