In a previous study it was found that with age there is an increase in the frequency of paranodal profiles of myelinated nerve fibers in the cerebral cortex of monkeys. This indicates that there is an increase in the number of internodal myelin segments, and raises the question of whether additional oligodendrocytes are necessary to generate the increased numbers of internodal myelin segments. The present study shows that in layer 4Cβ of monkey primary visual cortex there is an age-related increase in the number of oligodendrocytes. When young (4–10 years of age) and old (25–35 years of age) monkeys are compared, the increase is found to be ∼50%, and it begins in middle age (12–19 years old). It is also shown that although there is no increase in the population of astrocytes in layer 4Cβ with age, there appears to be a slight increase in the frequency of microglial cells. As their numbers increase, oligodendrocytes in pairs, rows and groups become more common, which suggests that additional oligodendrocytes are being generated by cell division. Since there is little evidence that mature oligodendrocytes can divide, it is probable that the new oligodendrocytes are generated from progenitor cells which, as many studies have shown, can be labeled by antibodies to NG2, a chondroitin sulfate proteoglycan. By comparing the appearance of these NG2-labeled cells with cells encountered in thin sections of normally prepared tissue, it is shown that the NG2-positive cells have the features of neuroglial cells that were previously described as β astrocytes.