Synaptic activity of the medial cerebellar nuclei was reversibly blocked in 6 Cebus monkeys by cooling through a sheath implanted alongside the fastigial nucleus. Such lesions produced in the dark a strong nystagmus (slow phase velocity 100-200 deg/sec). The slow phase of nystagmus was predominantly in the horizontal plane and was towards the side of the lesion (ipsilateral drift). The maximum velocity of drift was independent of eye position and was directly related to the degree of cooling. Vision abolished the nystagmus. If lights were turned on during nystagmus the drift velocity rapidly decreased to zero with an instantaneous and an exponential component. It is suggested that these results emphasize the importance of the medial cerebellum, possibly by way of the fastigial nucleus, in balancing the output of the paired vestibular nuclei.