After implantation of solid pieces of cerebellar primordia from 12-day-old C57BL embryos into the cerebellar parenchyma of 3- to 4-month-old "Purkinje cell degeneration" mutant mice, Purkinje cells from the donor leave the implant and differentiate while migrating into the host molecular layer. Electrophysiological studies were performed using in vitro cerebellar slice preparations from "Purkinje cell degeneration" mutants 1-2 months after grafting, when grafted Purkinje cells have reached their final location in the host molecular layer and have completed their morphological differentiation. Intracellular recordings obtained from 45 Purkinje cells in mutant mice demonstrated that such grafted neurons have normal bioelectrical properties including sodium and calcium conductances and inward rectification. Moreover, all grafted Purkinje cells responded to electrical white matter stimulation by a typical all-or-none climbing fiber response. Responses mediated through the activation of mossy and parallel fibers, as well as inhibitory postsynaptic potentials, were also recorded in a significant number of grafted Purkinje cells. On the whole, all these excitatory and inhibitory responses in grafted "Purkinje cell degeneration" mutant mice have characteristics comparable to those in control mice. After electrophysiological studies, Purkinje cells were further characterized by their positive staining by calbindin antibody. Neurons of this class were dispersed throughout the molecular layer of the host folia in which the electrophysiological recordings had been performed. The ectopic location of their perikarya, the presence of dendritic trees spanning most of the molecular layer (without entering the granular layer), and the occasional presence of axons emerging from the ectopic neurons and forming loose bundles at the white matter axis of the folia, corroborate the grafted nature of the Purkinje cells studied. Therefore, these experiments demonstrate that embryonic Purkinje cells from the graft can complete differentiation in the adult host cerebellum, and establish specific synaptic contacts with the presynaptic elements previously impinging on the missing neurons of "Purkinje cell degeneration" mutants. This process leads to a qualitative functional synaptic restoration of the cortical cerebellar network.