Multiple recordings from Purkinje cells in the rat cerebellum allowed the mechanism responsible for the activation of rows of synchronous complex spikes to be investigated. By determining the spatial distribution of the climbing fibre reflex that follows electrical microstimulation of the cerebellar cortex, it was shown that the mechanism for the simultaneity of firing was the electrotonic interactions between neurons in the inferior olive (IO). The spatial organization of the complex spike activity was shown to be regulated by GABAergic inhibitory input into the IO, probably arising from the cerebellar nuclear neurons. The rostro-caudal organizion of the complex spike activity following physiological stimulation (tactile stimulation of the upper and lower lip) demonstrated the same spatial distribution of synchronous activity in the cerebellar cortex as did the spontaneous activity and this was also disrupted by GABA blockers. Finally, complex spike responses to physiological stimulation indicate that the IO is capable of gating sensory inputs in accordance with its intrinsic autorhythmicity and that strong peripheral stimuli reset the oscillatory properties of the IO. The functional implications of the synchronicity and of the temporo-spatial organizion of complex spikes in the cerebellar cortex are discussed in the context of motor coordination and timing.