The distribution of serotonin neurons in the mouse brain was studied using a peroxidase-antiperoxidase (PAP) immunohistochemical method to obtain quantitative data on serotonin-containing cell bodies. The average total number of perikarya of serotonin neurons was about 26,000, of which about 17,000 cells (66.9%) were situated in the raphe system. The nucleus raphe dorsalis was the main region where a large number of serotonin neurons were present, and it contained about 9000 cells. Other than the raphe system, serotonin-immunoreactive perikarya were seen in the following regions: the reticular formation (18.9%), the lemniscus medialis (8.5%), the nucleus interpeduncularis (1.5%), the subpyramidal region (4.0%) and the periaqueductal gray (0.2%).