Extracellular records were made of single unit activity evoked in the fastigial nucleus (FN) by electrical stimulation of the caudate nucleus (CN) in cats anaesthetized with sodium thiopentone. 2. Single shock stimulation evoked bilaterally complex responses having up to three components. These were, in temporal order, with percent of units exhibiting them, (a) a short burst of evoked spikes with either a latency of smaller than 5-5 msec and no associated field potential (6 percent) or a latency of 7-5-20 msec and associated with a prominent negative-going field potential (36 percent); (b) suppression of spontaneous discharges for a period of 20-150 msec (90 percent); (c) a resumption of spike discharges with a transient increase in frequency lasting for 25-500 msec (66 percent). 3. Changes in component (c) of the response patterns of some units were noted during repetitive stimulation The nature of the change depended on the laterality of the FN with respect to the stimulated CN. 4. Mechanisms which might account for the responses are discussed, but it is emphasized that some of the results cannot yet be satisfactorily explained.
