The number and size of myelinated axons in the L₄ dorsal spinal root were determined in young rats and compared to those in the adult (older than 1 year) and in the old rats (older than 2 years). While the total number of myelinated axons was about the same at all ages, the average overall axon diameter showed a growth-related increase in adult animals, followed by a decrease reflecting senile atrophy in old rats. It is suggested that in aging laboratory rats the extent of axon shrinkage could account for the occurrence of secondary radicular demyelination.