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1. Responses of the cerebellar cortex in anaesthetized cats were evoked by mossy fibre and/or climbing fibre inputs, and the effects of graded cooling of the cerebellar cortex were investigated. Cooling was applied either globally by flooding the exposed cortex with cooled Ringer Locke, or in later experiments locally be passing cooled fluid through a silver tube in contact with the cerebellar cortex. The cortical temperature was continuously monitored by a thermistor inserted to a depth of 0.5 mm close to the recording site. 2. In the granular layer the cooling caused a large increase in the diphasic P1N1 wave generated by the afferent mossy fibre volley. The waves generated by synaptic excitation and discharge of granule cells, N2P2, were not diminished until the temperature fell towards 20 degrees C. In contrast the N3 wave of the molecular layer was largest with cooling in the range of 35 to 25 degrees C, often several times larger than at 38 to 40 degrees C. Associated with the enhanced N3 wave there was an enhanced N4 wave, which indicates an increased discharge by Purkyn cells. 3. Climbing fibre inputs generate a negative field potential in the molecular layer due to the powerful excitation of Purkyn cells. In contrast to the N3 potential this climbing fibre wave was largest at the higher temperatures 35-40 degrees C and declined progressively with cooling, being usually suppressed at moderate coolings of 31-27 degrees C. Intracellular recording revealed that the diminution was due both to the elimination of all but the first impulses of the normal burst discharge of the climbing fibre impulses and to the diminution of the synaptic excitation of a single climbing fibre impulse. 4. It is shown that the negative potentials produced in the molecular layer by combinations of mossy fibre and climbing fibre inputs can be very effectively distinguished by this differential effect of cooling. 5. The effects of cooling even to a severe level are immediately recoverable on warming. Repeated cooling has no untoward effects and there is no sign of the hysteresis reported for the cuneate nucleus. 6. There is a discussion of the factors that could cause cooling to differentiate between the actions of the mossy fibre and climbing fibre impulses on Purkyn cells.