We have investigated the long-term structural changes which affect Purkinje cells exposed to a single dose of kainic acid. Following intraparenchymal injection of the excitotoxin in the cerebellar cortex (1 microliter of a 1 mg/ml solution), Purkinje cells which survived within the lesioned area or close to its edges showed remarkable axonal abnormalities, involving the formation of torpedoes, hypertrophy of recurrent collaterals and atrophy of the corticofugal portion of the axon. In addition, their dendritic trees were often affected by conspicuous regressive alterations. The climbing fibres contacting these Purkinje cells were characterized by thick perisomatic plexuses, whereas their peridendritic branches were atrophic. The dendrites innervated by such atrophic olivary arbours were studded with huge numbers of newly formed spines. These alterations were already present a few days after kainic acid administration and persisted for the total period of observation of 6 months after the lesion. The remarkable similarity between the abnormalities of Purkinje cells exposed to kainic acid and those observed after axotomy indicates that in these two conditions common mechanisms determine analogous long-lasting modifications in the affected neurons. It is proposed that kainic acid-induced intracellular calcium overload disrupts cytoskeletal components and impairs axonal transport, thus depriving the affected Purkinje cells of retrograde trophic influences from their target neurons. As a consequence the affected neurons undergo long-lasting regressive modifications and compensatory remodelling phenomena.