The fine structure of neuronal somata and axosomatic synapses in each subnucleus of Gudden's tegmental nuclei was studied by use of electron microscopy. The pars principalis of the ventral tegmental nucleus of Gudden (TVP) is composed of oblong or triangular, medium sized neurons (11.8 x 22.6 microns, 211.4 microns 2) containing many mitochondria, lysosomes, Golgi apparatus, and rough endoplasmic reticulum composing Nissl bodies. The light oval nucleus with a prominent nucleolus is centrally situated, and indentations of its nuclear envelope are recognized in all neurons. The neuron in the pars ventralis of the dorsal tegmental nucleus of Gudden (TDV) is similar to that in the TVP, but its average size is significantly smaller (10.0 x 18.8 microns, 151.4 microns 2), and its organelles are also less well developed. The pars dorsalis of the dorsal tegmental nucleus of Gudden (TDD) is composed of spindle shaped, small neurons (6.9 x 16.2 microns, 85.1 microns 2) characterized by their irregular shaped nucleus with its invaginated envelope. These neurons have a thin rim of cytoplasm, poorly developed organelles and no Nissl bodies. The average number of axosomatic terminals in a sectional plane is 9.9 in the TVP, 9.6 in the TDV and 2.6 in the TDD, and the bouton covering ratio is 24.3% in the TVP, 26.5% in the TDV and 7.4% in the TDD. The respective percentages of round, flat and pleomorphic type axosomatic terminals were estimated, and the flat type terminals were found to be dominant in the TVP, the pleomorphic type terminals in the TDV, and the round type terminals in the TDD.