In this study, evidence is presented that the electron-dense dot observed in the synaptic vesicles of axonal varicosities belonging to the supra-ependymal nerve fibres (SEF) of rats is related to the serotonin content of these vesicles. 2. The size of the dense dot and the relative number of small granular vesicles (SGV) were markedly increased by intraventricular administration of serotonin as well as by acute or chronic treatment with inhibitors of monoamine oxidases. 3. Conversely, the administration of reserpine diminished the percentage of SGV, as compared to untreated animals, and prevented the formation of granules in synaptic vesicles of chronic nialamide-treated rats. 4. With respect to large granular vesicles (LGV), it was found that chronic inhibition of MAO increased their number within the varicosities, whereas reserpine administration in untreated rats resulted in their disappearance. 5. Our results strongly imply that serotonin is present in both SGV and LGV of the SEF varicosities. This conclusion is supported by some radioautographic pictures obtained after intraventricular administration of 3H-5-HT which show silver grains over both populations of vesicles. Further support was obtained by the use of the Tranzer's cytochemical method in either untreated or nialamide treated rats. This method demonstrates that the reaction specific of monoamines occurs in both types of vesicles with either treatment but that larger stores of 5 HT (larger granules) are demonstrated after inhibition of MAO. 6. It is concluded that both SGV and LGV may represent storage organelles for exogenous and endogenous 5-HT.
