The number, size, density and pathologic alterations of myelinated fibers (MF) of ventral and dorsal roots and of peroneal and sural nerves in groups of female Fischer 344 rats at 10, 20, and 30 months of age were evaluated to characterize nerve changes with old age. Except for minimal changes in the peroneal nerve, no statistically significant alteration in number of MF/nerve or in the fascicular area was associated with aging. The unaltered number was misleading since striking changes in MF size distribution, pathologic alterations of fibers and the presence of regeneration clusters suggested age-related degenerative and regenerative events. These changes were most dramatic in the ventral root, where myelin infolding, myelin separation from axon and ballooning, macrophagia and hyperplasia of Schwann cell nuclei were pronounced. Concomitant with these alterations, axonal atrophy (a reduction in caliber of axons relative to myelin spiral length or number of myelin lamellae) was demonstrated in MF of the ventral root in old age.