Investigations of the neuronal organization of the vestibuloocular reflex are carried out in alert monkeys implanted with a scleral search coil for the accurate registration of eye movements. Single-neuron recordings are made with chronic techniques in brainstem regions involved in the operation of the reflex. Particular attention is focused on tracing the pathway mediating the apparent visual response in vestibular nucleus neurons. Neural sugstrates being investigated are the pretectal region, the nucleus reticularis tegmenti pontis, the sub-parafasiscularis region and the medial pontine reticular formation. In addition, the adaptive effect on single neurons of the vestibuloocular pathways of longer term alteration of the visual input during vestibular stimulation is investigated. Finally, the characteristics of adaptive visual modification of the vestibuloocular reflex is studied in animals which have been depleted of cerebellar catecholamine stores.