The fine structure of cells in the trigeminal mesencephalic (Me5) nucleus in the rat was studied by transmission electron microscopy. This nucleus located in the mid-brain and the rostral portion of the pons, most neurons in the caudal part of Me5 nucleus were sufficiently large (40~50micrometer) and appeared as round-shaped unipolar cells. The Me5 neurons had a large, round, centrally located nucleus, and their cytoplasm contained numerous mitochondria, dense granular endoplasmic reticulum, abundant Golgi apparatus, groups of free ribosomes, some heterogeneous particles and neurofilaments. Cell surfaces were studded irregularly with small spinouts processes, and they contained a few fine irregularly arranged neurofilaments and some granular endoplasmic reticulum. Boutons contacting the soma of Me5 neurons were defined as axosomatic synapses and bostons contacting dendrites located between the Me5 neurons were defined as axodendritic synapses. Based on differences in bouton and vesicle morphology, the four synaptic bouton types were identified. 1. Asymmetrical as well as symmetrical synapses with small round vesicles. 2. Asymmetrical synapses with pleomorphic admixture contained predominantlyspherical vesicles. 3. Symmetrical synapses with pleomorphic vesicles of flattened, spherical and dense-core vesicles. 4. Asymmetrical as well as symmetrical synapses with heterogeneous and large dense-core vesicles. Synaptic boutons containing round vesicles and large dense-core vesicles were most frequently observed.