Rhesus monkeys discriminated between two color-differentiated stimuli presented on a screen only when they fixated the center of the screen. The stimuli were presented 8 degrees, 20 degrees, or 32 degrees from the screen's center, for 2 sec or 100 msec, a duration too brief to permit their fixation. Performance declined when the response sites, located either centrally (8 degrees eccentricity) or peripherally (32 degrees eccentricity), were separated from the stimuli, whether they were presented for 2 sec or 100 msec. These findings suggest that the stimulus-response separation effect is due to selective attention to the response sites and not to fixating them during stimulus presentation. Following superior colliculus lesions, the monkeys were impaired in discriminating between peripheral stimuli, but only when they responded centrally. This deficit was not due to (a) a failure to fixate the stimuli, for it occurred when the stimuli were 100 msec or 2 sec or (b) reduced sensory capacities, for it disappeared when the subjects responded peripherally. This deficit may reflect deficient attentional shifts from the response sites to the stimuli.