Golgi cells are inhibitory interneurones located in the granular layer of the cerebellar cortex. They occupy a unique position in the circuitry of the cerebellum, receiving direct excitatory inputs from mossy fibres, and indirectly, by way of the mossy fibre-granule cell-parallel fibre pathway. Mossy fibres are the most numerous source of inputs to the cerebellum, and can project bilaterally to the cerebellum. Such bilaterality is likely to have a significant impact on cerebellar cortical function as sensory events may directly influence the processing of information on both sides of the cerebellum. Here, we use a double retrograde axonal tracing strategy to chart the possible sources of mossy fibres with bilateral inputs to putative Golgi cells (pGCs) by injecting red and green fluorescent latex microspheres into areas on both sides of the cerebellar cortex which are functionally linked. In adult rats anaesthetised with pentobarbitone (40mg/kg, i.p.) using stainless steel electrodes (2–3 MΩ) we have made extracellular recordings from pGCs located in the cerebellar hemisphere (Crus II) on one side. A monopolar stimulation electrode (0.1 MΩ) was used to stimulate the deep granular layers/white matter in the contralateral cerebellar hemisphere (0.5–1 mm deep). Systematic probing (0.5 mm intervals) of the contralateral cerebellar cortex with the stimulating electrode revealed areas of cortex from which single, post-synaptically evoked spikes could be observed in the recorded pGC (2–3 ms latency). The current required to evoke such spikes varied as a function of the location of the stimulating electrode and the firing rate of the recorded pGC. A microinjection of one colour of tracer material was made at the cortical site where the lowest intensity of stimulation evoked a response (10–30 μA), and a microinjection of the other colour tracer was made at the recording site on the other side of the cerebellum (~50 –100 nl of tracer in each microinjection). After 7 days survival for retrograde axonal transport of tracer to occur, the animals were anaesthetized with urethane (1000 mg/kg i.p.) and perfusion fixed. An epifluorescence microscope was used to survey various cerebellar and brainstem structures for retrogradely labelled cells (e.g. pontine nuclei, inferior olive, lateral reticular nucleus, cerebellar nuclei). Preliminary results indicate that double labelled cells are only present within the basilar pontine nucleus. This suggests that the pons may be the principal source of mossy fibres that branch to influence Golgi cells located in both cerebellar hemispheres.