Cells from the fetal central nervous system (CNS) of rat embryos survive and differentiate when transplanted into the peripheral nervous system (PNS) of adult rats. The experiments described here were aimed at investigating selected molecular and ultrastructural features of dissociated CNS cells from the telencephalon of 12-day-old embryos isolated for long periods of time within PNS segments. Neurons and glia of grafts examined 6-12 months after transplantation into the PNS developed several cytoskeletal abnormalities. In neurons, these changes included Hirano bodies within dendrites and a marked perikaryal immunoreactivity to RT97, a monoclonal antibody that normally recognizes in neuronal processes the phosphorylated 200 kDa protein subunit of neurofilaments. Rosenthal fibres were seen within the glial cells. Similar-looking abnormalities have been described in certain human and animal neurodegenerative diseases and in ageing. Although a relationship between the changes in these long-term neural transplants and such diseases is unknown, these observations provide an opportunity for studying their pathogenesis within laboratory conditions.