As part of a continuing study of the relationship between age and neuronal number within the human brainstem, the abducens nucleus was examined in a total of 20 brains. While the nucleus almost doubles in length between birth and adulthood; there is no significant change in cell number after birth. With the exception of some specimens which may indicate an individual variation, the average number of cells in this nucleus for all specimens examined was found to be 6,454. Approximately the same number is found in the nucleus of the newborn (6,500); confined within a length of 1.5 mm. With maturity, the nucleus enlarges to 3.0 mm but since the cell number does not significantly change, the cell density in any one section of the nucleus is considerably decreased, thus emphasizing the importance of absolute counts in such studies. At least among the brainstem nuclei already examined by various investigators it appears that neuronal loss is not a concomitant of aging as has been described for the cerebral and cerebellar cortices.