We describe an attempt to model unilateral vestibular dysfunction in normal man by inducing vestibular asymmetry with exposure to long-term, unidirectional, visual-vestibular conflict. Subjects were exposed to pseudo-random (0.13, 0.2, 0.25, 0.3Hz; 77 degrees/s peak) oscillation in Yaw for 30 min whilst viewing a surrounding, whole field optokinetic drum which rotated with them when they were rotating rightwards and remained earth stationary when they rotated leftwards. Adaptation to this stimulus was assessed by combined tests of "goal-directed" vestibular-ocular reflex (VOR) and vestibular memory contingent saccades (VMCS) in 5 subjects and in a further 4 subjects by combined tests of perception of reorientation (a "navigation" task) and sinusoidal VOR at 0.1 and 0.32 Hz. The exposure induced a reduction in the gain of the VMCS and an underestimation of perceived amplitude of displacement when subjects were turned rightwards. VOR gain for rightwards movement was reduced more markedly at 0.1 Hz. No change was found in the goal-directed VOR gain. Thirty minutes after adaptation, the asymmetry of the VOR gain remained at 0.1 Hz, but vestibular perception recovered to normal. Asymmetrical adaptation can be achieved with short exposures and is more marked for low frequency stimuli. Modification reflex of vestibular functions endures longer than of perception of reorientation.