The subcellular morphology of the mesencephalic trigeminal (Me5) nucleus in the rat was studied by transmission electron microscopy. Most neurons in the thin rostral as well as in the major caudal part of Me5 appeared as large (40-50 microns), round- to ovoid-shaped unipolar cells. A few neurons (estimated 5%) appeared to be multipolar, usually bipolar. The Me5 neurons had a large, round, centrally located nucleus, and their cytoplasm was characterized by a dense network of lamellar granular endoplasmic reticulum, an abundant Golgi apparatus, many mitochondria and neurofilaments suggesting very active cells with a high rate of synthesis and axoplasmatic transport. Numerous small spinous processes covered the surface of the Me5 neurons. Clustering of 2 or 3 cells was accomplished by maculae, i.e. zones of gap junctions and close cell appositions. Boutons contacting the soma of Me5 neurons and boutons contacting large and small dendrites were defined as axosomatic and axodendritic synapses, respectively. Four types of synaptic boutons were distinguished: (1) S boutons, with round vesicles and asymmetrical as well as symmetrical synapses, (2) F boutons, with pleomorphic admixture of flattened and spherical vesicles and asymmetrical synapses, (3) P boutons, which resembled the F-type boutons but contained predominantly spherical vesicles and symmetrical synapses, and (4) G boutons, characterized by a heterogeneous population of vesicles. This description of the Me5 nucleus is particularly useful for future studies that attempt to correlate the structure of a particular synapse with its function.