The vertical optokinetic nystagmus (OKN) of 10 normal subjects and the optokinetic afternystagmus (OKAN) of 3 subjects were measured with the magnetic search coil technique. In order to assess the relative contributions of various retinal areas to the up-down asymmetry in OKN the central and peripheral visual fields were selectively stimulated in four OKN conditions. In the full-field OKN condition the stimulus was a 61 degrees x 64 degrees display of moving random-dots. Overall, full-field OKN gains elicited by upward motion were significantly higher than those elicited by downward motion at stimulus velocities between 30 and 70 degrees/s. In the periphery-only OKN condition a 3 degrees or 6 degrees-wide vertical band occluded the center of the full-field display. Nine of the 10 subjects displayed OKN in this condition. For 6 subjects, the addition of the 6 degrees band to the full field resulted in an increase in the up-down asymmetry at stimulus velocities above 30 degrees/s. For the other three subjects there was a decline in the gains of both upward and downward OKN when the 3 degrees or 6 degrees band was present; the result was directionally symmetric OKN gains. In the central-strip OKN condition only a 6 degrees-wide central vertical strip of moving dots was visible. The gains of central-strip OKN were not significantly different from the full-field responses. A servo controlled centrally-located 10 degrees x 6 degrees moving display was used in the center-only OKN condition. In this condition both upward and downward gains were attenuated and there was no up-down asymmetry. OKAN was measured following a 50-s exposure to either the full-field or center-only OKN display. The stimulus velocity was 30 degrees/s. After viewing the full-field display the 3 subjects displayed OKAN with slow phases upward following upward OKN but there was no downward OKAN following downward OKN. In contrast, there was no consistent directional asymmetry following exposure to the center-only display. The disappearance of the upward preponderance in OKN and OKAN with occlusion of the peripheral retina suggests that the directional asymmetry in vertical OKN exists in the slow OKN system.