Inhibition of cerebellar granule cells (GrCs) by Golgi cells (GoCs) is thought to be important for cerebellar information processing (Marr, 1969; Wantanabe et al. 1999). GoCs receive excitatory input from mossy fibres (MFs) and parallel fibres, an anatomical arrangement that provides both feed-forward and feed-back inhibition onto GrCs. We have investigated the properties of putative MF synaptic connections onto GoCs and examined their impact on GoC firing. Excitatory postsynaptic currents (EPSCs) evoked with white matter stimulation were recorded using whole-cell voltage-clamp recording of GoCs in cerebellar slices from p25 rats at 35°C. EPSCs evoked with minimal stimulation had fast kinetics (10-90% rise time=0.3±0.1ms; 37% decay time=0.9±0.4ms) and a mean amplitude of 57±25pA with high variability (CV=0.72; n=18). The short response latency (0.8±0.2ms), pharmacological profile and absence of pronounced short-term plasticity suggest that EPSCs result from MF stimulation. To examine the role of the putative MF input, we recorded GoC firing in response to MF tract stimulation in loose cell-attached patch mode. GoCs were spontaneously active at 9±4Hz. Single-pulse MF tract stimulation elicited an action potential (AP), that reset the phase of spontaneous GoC firing. Subsequent whole-cell recordings at the same stimulus intensity revealed that the synaptic current underlying AP generation was 230±110pA; n=5. This suggests that simultaneous activation of a few MFs is sufficient to reset GoC firing. Our data provide information on the properties of the previously uncharacterized MF-GoC synaptic connection and suggests that MFs have a high efficacy in driving GoC firing. Our results suggest that simultaneous activation of few MFs can influence the timing of inhibition onto the ~5000 GrCs innervated by a single GoC axon (Pellionisz and Szentagothai in Ito 1984). This feed-forward inhibition therefore is likely to play an important role in signal processing in the granule cell layer by synchronizing the time window of synaptic integration in granule cells.