1. A single climbing fibre makes an extraordinarily extensive synaptic contact with the dendrites of a Purkinje cell. Investigation of this synaptic mechanism in the cerebellum of the cat has been based on the discovery by Szentagothai & Rajkovits (1959) that the climbing fibres have their cells of origin in the contralateral inferior olive.2. Stimulation in the accessory olive selectively excites fibres that have a powerful synaptic excitatory action on Purkinje cells in the contralateral vermis, evoking a repetitive spike discharge of 5-7 msec duration. Almost invariably this response had an all-or-nothing character. In every respect it corresponds with the synaptic action that is to be expected from climbing fibres.3. Intracellular recording from Purkinje cells reveals that this climbing fibre stimulation evokes a large unitary depolarization with an initial spike and later partial spike responses superimposed on a sustained depolarization.4. Typical climbing fibre responses can be excited, but in a much less selective manner, by stimulation of the olive-cerebellar pathway in the region of the fastigial nucleus, there being often a preceding antidromic spike potential of the Purkinje cell under observation.5. Impaled Purkinje cells rapidly deteriorate with loss of all spike discharge, the climbing fibre response being then reduced to an excitatory post-synaptic potential. This potential shows that stimulation of the inferior olive may evoke two or more discharges at about 2 msec intervals in the same climbing fibre. The complexity of neuronal connexions in the inferior olive is also indicated by the considerable latency range in responses.6. A further complication is that, with stimulation in the region of the fastigial nucleus, the initial direct climbing fibre response is often followed by a reflex discharge, presumably from the inferior olive, which resembles the responses produced by inferior olive stimulation in being often repetitive.7. Typical climbing fibre responses have been evoked by peripheral nerve stimulation and frequently occur spontaneously.8. An account is given of the way in which the responses evoked by climbing fibres in the individual Purkinje cells can account for the potential fields that an inferior olive stimulus evokes on the surface and through the depth of the cerebellar cortex.9. By the application of currents through the recording intracellular electrode it has been possible to effect large changes in the excitatory post-synaptic potential produced by a climbing fibre, it being diminished and even reversed with depolarizing currents and greatly increased by hyperpolarizing currents.