1. The gain of the vestibuloocular reflex (slow-phase eye velocity/head velocity) was measured in 17 adult cats. 2. The gain of the reflex, in the dark, was 0.90 (+/-0.15 SD) over the frequency range 0.03-1.2 Hz. 3. In the range 0.01-0.15 Hz, the phase behaved as though the overall reflex time constant were 12 s or greater. The cupula time constant is 4 s. Therefore, the central part of the reflex must manipulate the canal signal to improve its low-frequency response by a factor of at least three. 4. When the cats wore left-right reversing prisms chronically and were also rotated for 2 h every day, the reflex underwent large, plastic changes. The gain, tested in the dark, decreased by 93% at 0.05 Hz and 55% at 1.2 Hz. In effect, the low-frequency response was abolished. The process took about 8 days. 5. In the light, with reversed vision, the gain decreased further and, at low frequencies, the eye movements did reverse in direction. 6. When the vestibulocerebellum was removed, the gain, in the dark, rose to about 1.17 and the plastic changes caused by reversing prisms were completely abolished. 7. Reversing prisms create vestibuloocular dysmetria. The change in gain they produce is considered to be an adaptive response designed to reduce image motion on the retina during head movements. The vestibulocerebellum is necessary for this adaptive process. It is proposed that detecting and repairing dysmetria (of natural origin) is an important cerebellar function.