Serotonin (5-HT)-containing nerve cells in the rabbit brain were visualized immunohistochemically and were mapped on photographs of coronal brain sections. The 5-HT cells extended from the caudal medulla oblongata to the rostral midbrain and were mostly clustered in groups resembling those found in the rat brainstem. However, there were several notable differences between the rabbit and the rat in the distribution of 5-HT nerve cells. In particular, there were fewer pontine 5-HT cells in the rabbit and the major 5-HT cell groups of the midbrain and the ventral medulla oblongata were spread further laterally than in the rat brain.