RÉSUMÉ
Serotonin (5-HT) neurons in the nucleus raphe magnus (NRM) of the rat were identified by light and electron microscope immunocytochemistry methods. Serotoninlike immunoreactive (5-HT-li) neurous were large multipolar and fusiform cells which mainly located in the ventrocaudal part of the NRM. With electron microscopic immunocytochemistry the following findings were revealed: 1. Immunostaining was present in the cytoplasmic matrix, outer membrane of mitochondria, rough endoplasmic reticulum, multivesicular bodies and vesicles. Some nuclei of the cells were stained. 2.5-HT-li somata, dendrites and axons were postsynaptic to numerous unlabeled terminals which contained clear round vesicles (20-30 nm) or mixed with small granular vesicles (40-70 nm), large granular vesicles (90-110 nm) and flat clear vesicles. 3. The 5-HT-li axons were unmyelinated fibers, and the 5-HT-li axon terminals were scarce in the NRM. 4.5-HT-li axon terminals and dendrites abutting on capillaries and 5-HT-li dendrite-glial direct appositions were found in the NRM.These results revealed the ultrastructural characteristics of 5-HT-li neuron in the NRM. The variety of unlabeled terminals making contact with 5-HT-li somata and dendrites suggests that several neuronal system with possibly different transmitters may regulate 5-HT raphe-spinal neurons which may play integrative role in the NRM. Tae relationship between 5-HT neuron and the significance of the local microcirculation and the relationship between 5-HT neurons and glia cells were also discussed in the present paper.