Molecular layers of the adult rat cerebellum, the inferior olivary neurones and climbing fibres which had been destroyed by 3-acetylpyridine (3-AP) and inferior cerebellar pedunculotomy, were examined using an antibody against corticotropin releasing factor (CRF) to visualize mossy fibres. Sprouting mossy fibres appeared in the molecular layer 2 days after 3-AP treatment. Morphologically, they showed no bundle formation, rosette-like swellings along their course, made ectopic synapses with interneurones, and had no association with arrested granule cells. The number of sprouting fibres gradually increased with time after 3-AP treatment. These findings indicate that mossy fibre sprouting is continuously induced in the climbing fibre-deficient cerebellum.