SUMMARY1. In cats anaesthetized with oc-chloralose, micro-electrodes have been used to record the discharge patterns of single neurones in the region of the nucleus interpositus.2. Almost all cells tested could be antidromically invaded following electrical stimulation of the contralateral red nucleus, showing that they were cerebellar efferent neurones.3. A little over half of the interpositus neurones were spontaneously active, usually at rates of less than 20 impulses/sec. 4. About 40 % of the cells had no spontaneous activity, although they gave brisk responses to electrical stimulation of cutaneous nerves. Such silent units were encountered most frequently in the earlier stages of an experiment, but a number were found more than 15 hr after the beginning of an experiment.5. Stimulation of cutaneous and mixed nerves of the fore and hind limbs provoked impulse discharges of the cells and also produced phases of deceleration of the resting discharge of spontaneously firing cells.6. The typical response of an interpositus neurone consisted of a short latency (6-35 msec) discharge, usually separated from a long latency (50-500 msec) discharge by a period of inhibition or return to the resting discharge rate. The two phases of excitation appeared to be independently generated, since in a number of cells one phase appeared without the other. In addition, the later phase of excitation was abolished in all cells tested by a small dose of pentobarbitone which produced very little effect on the earlier phase. The 9. The possibility that these long latency responses have a physiological significance in relation to locomotion is discussed.

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SUMMARY1. Micro-electrodes have been used to record from 119 Purkinje (P) cells in the paramedian lobule of the cerebellum in cats anaesthetized with c-chloralose.2. The spontaneous discharge rate and degree of irregularity of the discharge varied very much from one cell to another; the over-all mean rate (about 25/sec) was a little lower than has been reported either for barbiturate anaesthetized or for decerebrate unanaesthetized preparations.3. Following electrical stimulation of a peripheral nerve, most P cells responded with both simple spikes and a climbing fibre response. This initial response was usually succeeded by a prolonged period of silence (over-all mean duration 350 msec) before resumption of the tonic simple spike discharge. Similar response-silence sequences could also be evoked by mechanical stimulation such as a tap applied to the pads of the forepaw.4, Electrical stimulation of the inferior olive evoked climbing fibre responses followed by a prolonged pause in the simple spike discharge of the cell.5. In six individual preparations, recordings were made both from P cells of the paramedian lobule and from neurones of nucleus interpositus (to which the former project). Comparison of the responses of the two types of neurone to peripheral nerve and inferior olivary stimulation showed that the end of the pauses in P cell firing correlated well with the end of a prolonged period of facilitation of the interpositus neurones.6. These results support the hypothesis advanced in an earlier report (Armstrong, Cogdell & Harvey, 1975) that the prolonged facilitatory responses of interpositus neurones are essentially disinhibitory responses resulting from reduction in the activity of overlying cells, and that responses of P cells and of interpositus neurones consist, in general, of modulations of activity which are mutually out of phase.