Cerebellar granule cell to Purkinje cell synapses have been reported to show plasticity when stimulating the parallel fibres, but not when granule cell axons are stimulated in the granular layer. The latter absence of plasticity has been attributed either to the synapses made by ascending granule cell axons lacking some feature needed to evoke plasticity, such as metabotropic glutamate receptors, or to spillover of glutamate between adjacent active synapses being essential for plasticity to occur and having a greater effect for parallel fibre stimulation than for granular layer stimulation. Here we show that both long-term depression (LTD) and endocannabinoid plasticity can depend on interaction between adjacent synapses. These results focus attention on the need to characterize the spatial pattern of parallel fibre activity evoked by physiological stimuli, in order to assess the conditions under which synaptic plasticity will occur in vivo.