Ultrastructural observations have been made on the tibial and plantar nerves of Wistar rats aged 18-24 months. Changes indicative of segmental demyelination and remyelination and axonal degeneration and regeneration were prominent in the plantar nerves. Both in the plantar and tibial nerves, but particularly in the former, axonal abnormalities were frequent. These included the occurrence of multiple intra-axonal vacuoles containing glycogen and polyglucosan bodies. Axonal sequestration by adaxonal Schwann cell processes was also increased. The Schwann cell cytoplasm in relation to this activity contained bundles of filaments with the ultrastructural features of Hirano bodies. The changes in the plantar nerves probably indicate a pressure neuropathy, but the possibility of a superimposed distal axonal degeneration related to aging cannot be excluded on the present evidence. Such changes must be taken into consideration in experimental studies performed on rats of this age.