The distribution of serotonin neuronal somata in the brain of the rabbit was demonstrated by use of immunoperoxidase technique. The serotonin antibody obtained from a rabbit was applied with the two-step ABC (avidin-biotin-peroxidase complex) method after biotinylation of this antibody. The location, shape, size and immunoreactivity of these somata were clarified in our permanent preparations without non-specific and background staining. These perikarya were distributed in the raphe nuclei,

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... ar nuclei and interpeduncular nucleus, within and around the medial lemniscus, in the mesencephalic periaqueductal grey lateral to the raphe dorsalis, and in the reticular formation of the brain stem. However the so-called "masked indolamine cells" were not observed in the hypothalamus or the area postrema, as our experimental animals were not given any pharmacological pretreatment. These results were essentially similar to those from other mammalian species, especially the rat.