Voluntary control of the human vestibulo-ocular reflex with and without visual targets was investigated. Subjects were rotated sinusoidally from 0.1 to 1.0 Hz using d.c. electro-oculography to record eye position. The ratio of eye to head movement, or gain, of the vestibulo-ocular reflex was measured. When subjects were rotated in the dark at 0.3 Hz whilst performing mental arithmetic the gain was 0.65. When subjects were asked to fixate imaginary targets in the dark that were stationary in space, the gain rose to 0.95. When they imagined targets rotating with them on the chair, the gain dropped to 0.35. Our results indicate that the ability to modulate the gain of the vestibulo-ocular reflex does not depend entirely on the smooth pursuit system. Higher centers must modulate eye velocity so that it is appropriate to the subject's choice of a frame of reference, whether or not vision is available.