1. Mossy fibre evoked activity in the cerebellar cortex elicited by peripheral electrical stimulation was studied in chloralose anesthetized cats. The distribution of intracortical field potentials in the C3 and D zones was mapped in order to determine if there is a spread of synaptic activity outside the mossy fibre termination area. This area was identified by the presence of short latency synaptic field potentials in the granular layer. 2. Molecular layer field potentials were recorded up to 1.5 mm outside the mossy fibre termination area. The latencies of these potentials increased with increasing distance from the mossy fibre termination area, corresponding to a conduction velocity of about 0.4 m/s. 3. Recordings from Purkinje cells, within and outside the mossy fibre termination area, revealed an increase of simple spike activity at latencies corresponding to those of the field potentials in the same location. 4. From the spatial and temporal characteristics of the evoked activity, it is concluded that a mossy fibre input results in spread of synaptic activity along the parallel fibres. 5. The findings are discussed in relation to the recently discovered microzonal organization of the C3 zone. It is proposed that the organization of this zone offers a possibility for the control of muscle synergies, each synergy being represented by a mossy fibre input and the specific set of microzones activated by this input via the parallel fibres.