Observations have been made on the changes in the myelinated fibres of the rat tibial and plantar nerves between 2 and 24 months of age. There is an initial rapid increase in fibre diameter followed by a later more gradual increase, which ceases after approximately 9 months of age in the tibial nerve but which continues for longer in the medial plantar nerve. The fibre size distribution remains substantially unimodal throughout. In both nerves maximal and average fibre diameter become reduced by 24 months. Total fibre number shows considerable variability between animals, but no definite systematic alteration with age is detectable. Teased fibre preparations demonstrate a low level of abnormality in the tibial nerve until after 18 months of age, but by 24 months approximately 30% of fibres display abnormalities. Although both paranodal and segmental demyelination and remyelination, and axonal degeneration and regeneration occur, the latter type of change predominates. By contrast, in the lateral plantar nerve paranodal and segmental demyelination become detectable to a significant extent from 6 months of age. Axonal degeneration and regeneration also become evident after 15 months, and by 24 months of age 55% of fibres show abnormalities. The possible explanation of these changes is discussed, as is their relevance to the frequent use of the tibial nerve for studies on experimental neuropathies.