Degenerative changes of the pars compacta of the substantia nigra are considered the main physiopathological basis of Parkinson''s disease, while most authors believe that the neostriatum is well preserved in these cases. This paper deals with the preliminary ultrastructural observations made in the neuropil of the caudate nucleus of Parkinson''s disease patients. We have observed (1) astrocytic proliferation, neuronal degeneration, degenerated axons, and hyperdense postsynaptic neurites (dendrites), and (2) that degenerative patterns vary from one case to another. Physiopathological and therapeutical implications are discussed.