1. To determine the possible role of the primate pretectal nucleus of the optic tract (NOT) in the generation of optokinetic and smooth-pursuit eye movements, we recorded the activity of 155 single units in four behaving rhesus macaques. The monkeys were trained to fixate a stationary target spot during visual testing and to track a small moving spot in a variety of visual environments. 2. The majority (82%) of NOT neurons responded only to visual stimuli. Most units responded vigorously for large-field (70 x 50 degrees) moving visual stimuli and responded less, if at all, during smooth-pursuit eye movements in the dark; many of these units had large receptive fields (greater than 10 x 10 degrees) that included the fovea. The remaining visual units responded more vigorously during smooth-pursuit eye movements in the dark than during movement of large-field visual stimuli; all but one had small receptive fields (less than 10 x 10 degrees) that included the fovea. For all visual units that responded during smooth pursuit, extinction of the small moving target so briefly that pursuit continued caused the firing rates to drop to resting levels, confirming that the discharge was due to visual stimulation of receptive fields with foveal and perifoveal movement sensitivity and not to smooth-pursuit eye movements per se. 3. Eighteen percent of all NOT units ceased their tonic discharge in association with all saccades including the quick phases accompanying optokinetic or vestibular nystagmus. The pause in firing began after saccade onset, was unrelated to saccade duration, and occurred even in complete darkness. 4. Most (90%) of the visual NOT units were direction selective. They exhibited an increase in firing above resting during horizontal (ipsilateral) background movement and/or during smooth pursuit of a moving spot and a decrease in firing during contralateral movement. 5. The firing rates of NOT units were highly dependent on stimulus velocity. All had velocity thresholds of less than 1 degree/s and exhibited a monotonic increase in firing rate with visual stimulus velocity over part (n = 90%) or all (n = 10%) of the tested range (i.e., 1-200 degrees/s). Most NOT units exhibited velocity tuning with an average preferred velocity of 64 degrees/s.(ABSTRACT TRUNCATED AT 400 WORDS)