The adult olivocerebellar axons and their terminal arbours, the climbing fibres, are capable of remarkable structural plasticity, regulated through their interaction with Purkinje cells. When these cells are deleted,terminal climbing fibre branches retract. In contrast,there is a vigorous outgrowth of entire terminal arbours when extra postsynaptic neurones are available. The new connections lead to a functional, highly specific pattern of innervation at the single Purkinje cell level and are topographically organized according to the principles of the original projection map.A reversible climbing fibre retraction occurs following depression of electrical activity of the cerebellar cortex. These remarkable plastic properties, together with the fact that these neurones express several growth-associated genes constitutively, suggest that the climbing fibre synapses might be adjusted dynamically to participate in physiological plasticity.