We have examined the ultrastructural changes of axons and synaptic boutons in the intracerebellar nuclei of the rat at 3 days to one year after inferior olive lesion performed by means of electrocoagulation or 3-acetylpyridine injection. A large number of preterminal segments and axons terminals undergoes remarkable ultrastructural changes after total or subtotal olivary lesion. Large membrane bound vacuoles and clusters of small synaptic vesicles characterize a good number of these terminals at 3 days up to one month after the lesion. Tightly packed tubules and cisternae of smooth endoplasmic reticulum appear during the first week in an increasing number of axon terminals. Boutons with large whorled bodies formed by smooth membranes increase in number during the second half of the first month and further increase in density until the sixth month. They are still present in large amounts at one year. Immunoreactivity for 3',5'-guanosine-phosphate-dependent protein kinase, which is specific for Purkinje neurons, can be detected in the axons and synaptic terminals displaying the ultrastructural changes described above. These results are discussed in relation to a possible trophic action of the climbing fibers on the Purkinje cells. We suggest that, at least in part, these alterations may be the consequence of the intense Purkinje cell hyperactivity which is present for up to one month from inferior olive lesion.