In the rhesus monkey primary visual cortex, there are bundles of vertically oriented myelinated axons, which mainly contain efferent fibers originating from pyramidal cells. At the level of layer 4Cbeta, the bundles are regularly arranged and the nerve fibers in them are closely packed. In order to determine if a significant loss of intracortical nerve fibers occurs as the primate cerebral cortex ages, the frequency of vertically oriented myelinated fibers was examined at the level of layer 4Cbeta in 1 microm-thick, tangential sections. The results show no statistically significant differences in the numbers of vertically oriented fibers beneath 1 mm(2) of cortical surface between young, middle-aged, and old monkeys, and electron microscopic examination reveals few signs of degenerating axons. There is, however, an age-related breakdown of the myelin in sheaths that surround some axons. Thus, the data indicate that there is not a loss of vertically oriented myelinated fibers from the cortical gray matter during aging, although their sheaths may be altered.