The cytology of the superior olivary complex was studied in nissl stained sections of eight human brainstems, including adult, infant and fetus, and in the brains of ten juvenile rhesus monkeys. The most prominent components of the superior olivary complex of primates were specifically investigated, i.e. the medial (SOM) and lateral (SOL) superior olivary nuclei. Cell counts of these segments were done in human brainstems. The adult SOM was comprised of an average of 11,428 (7,850--15,010) perikarya; the SOL contained an average of 3,923 (2,890--5,400) neurons. These findings indicate that the SOL contains as many cells as reported in other primates, and is not reduced. The SOL appears somewhat inconspicuous in the human because it is organized into as many as six clusters of cells rather than forming one well circumscribed configuration as in the monkey and cat. The total cell population of the SOM together with the SOL was approximately the same on each side of individual brains. If one segment was larger on one side than the opposite side, the other segment was correspondingly reduced to maintain the relative symmetry. This suggests that a single mechanism controls the cell complement of at least two segments of the superior olivary complex.