Using ultrastructural methods degenerative changes were examined in 16 ataxic (ux/ux) rabbits of The Jackson Laboratory strain. These animals were killed between 4 and 53 days after the onset of symptoms using arterial perfusion of a paraformaldehyde-glutaraldehyde fixative made up in cacodylate buffer. Ten age matched controls were also studied.Alterations were observed in both neuropil and neurones of the involved nuclei, those in neuropil ordinarily remaining more significant throughout. The neuropil involvement focused upon astrocytes and axon terminals, the astrocytes showing both oedematous swelling and glycogen accumulation, the terminals secondary degeneration of acute and chronic types. Over time the coalescence of swollen glial processes leads to the formation of fluid filled extracellular spaces which appear to result from rupture of astrocytic membranes. That, together with the splitting of the lamellae of myelinated axons to form other lacunae, gives to the neuropil a progressively more spongioform appearance. With the disappearance of degenerating boutons, that vacuolation leads ultimately to loss of the glial investitures of the contained neurones.Large and medium-sized nerve cells can become involved in either acute or chronic cell change. Nissl substance may undergo compaction around the nucleus, the cytoplasm becoming quite fibrillar with reduction or loss of organelles. Mitochondrial alteration may be the only marker of an involved element. Cell parts in advanced degeneration are wrapped by processes of scavenger cells and removed.Lysosomes are found in excess in degenerating nerve cells, in astrocytes and in macrophages.