As a part of a continuing study of the relationship between age and neuronal population within the human brain stem, the nucleus of the trochlear nerve was examined in a total of 20 brains from newborn to 87 years of age. The cell count in the newborn was 1,830, and in the 87-year-old 2,010, while the mean number for all the specimens examined was 2,115. It is apparent that there is no cell loss in this nucleus in the normal aging process, which is in accord with the findings in five other brain stem nuclei examined by various investigators. This is in contrast to the findings in cerebral and cerebellar cortex which show significant cell loss with advancing age. Based on this and previous studies of the brain stem nuclei, it appears that neuronal loss is not a concomitant of the normal aging process in the brain stem. The anatomical basis of this selectivity in neuronal loss with aging in certain parts of the brain is not known at present.