In goldfish, adaptive gain control of the vestibuloocular reflex (VOR) is blocked by cerebellectomy. The operation was rapidly performed on alert goldfish before and after extended periods of adaptive gain training of the VOR produced by sinusoidal oscillation in the horizontal plane. The VOR in these conditions was abolished by sectioning the horizontal semicircular canals. Removal of the cerebellum from naive goldfish resulted in VOR gains significantly greater than 1 at all frequencies tested, with an average value near 1.4 at 1/8 Hz. This value represents an increase of about 65% over the initial VOR gain of 0.85. Changes in phase of the reflex were negligible. Cerebellectomy in animals previously trained to higher or lower gains immediately produced the same mean gain as in cerebellectomized naive animals; gains were increased in animals trained to lower gains and decreased in animals trained to higher gains. As little as 1 min separated aspiration and subsequent gain measurements. These results suggest that the cerebellum not only acts on extra cerebellar circuitry during the training, but that it is also involved in retaining the altered VOR gain. Adaptive gain control could not be achieved with prolonged training after cerebellectomy; in addition, cerebellectomy did not affect the response to visual stimulation at the onset of training to decrease or increase gain.