The use of different types of 3H-5 HT administration (intraventricular and subarachnoïdal injections) has allowed to detect, in addition to the well-known serotonergic neurons, new labelled neuronal cell bodies in various areas of the brain: nuclei pontis (lateralis and pars medialis), nucleus interfascicularis, nucleus paranigralis, nucleus tegmentalis laterodorsalis and among the cells of the locus coeruleus. Moreover, a preliminary study shows that, in the lenniscus medialis, nucleus interpreduncularis paramedialis and pontine nuclei, more than 50% of the labelled, presumably serotonergic cells are in direct contact with blood vessels, while in the raphe dorsalis and nucleus interpeduncularis dorsalis the percentage is only 20 to 25%. Possible implications of these findings are discussed in terms of the nature of labelled neurons and of their eventual physiological function.