The localization of serotonin(5HT)-containing subependymal cells was demonstrated by immunochemistry. The 5HT-containing subependymal cells were predominantly located in the paraventricular organ, while a small number of them were situated in the nucleus infundibularis dorsalis. The basal processes of these 5HT cells of the paraventricular organs appeared to have contact with the wall of blood vessels. The result was discussed in comparison with that obtained by the formaldehyde induced fluorescence (FIF) method.