Optokinetic nystagmus (OKN) was elicited in humans by a horizontally moving grating covering the whole visual field. Selective stimulation of central or peripheral parts of the retina was achieved by partial masking or scotopic viewing conditions in normals; three patients with a unilateral central retinal scotoma were studied in addition. In all cases, the elimination of foveal stimulation was accompanied by a decrease in OKN slow phase velocity compared to whole field stimulation. Vertical masks with retinally stabilized edges were used to selectively occlude or stimulate central or peripheral sectors with a fixed retinal location. A central stimulus was always more effective than the complementary peripheral stimulus, until the central zone was narrowed down to a width of 5-10 degrees. This central dominance was found throughout the range of velocities (6-180 degrees/s) and spatial frequencies (0.05-0.5 cycles/deg) used. A horizontal central band of occlusion caused a smaller decrease of OKN than a vertical occlusion with the same width. Scotopic vision caused a uniform mild decrease in OKN gain throughout the velocity range, provided that the spatio-temporal frequency of the stimulus remained within the scotopic resolution range. The patients had a slightly lower OKN gain when viewing with the scotomatous eye than with the contralateral, normal eye. The normal slight preference for temporal-to-nasal motion was not accentuated by masking or scotopic vision in normal eyes, but was enhanced in the eyes with the pathological scotomata in two of the three patients. All responses were immediate; no slow build-up was seen under any condition.