The frontal ganglion, part of the stomatogastric nervous system, contains about 60 to 80 neurons, 25 to 30 micron in diameter. A well developed Golgi system, producing dense-core vesicles, lysosomes, multivesicular bodies and dense bodies are abundant. Glia elements are sparsely distributed. Many nerve fibres contain granules of different size and electron density. Five groups of fibres can be distinguished: Fibres with granules of about 200 nm (type A), fibres with granules of about 160 to 170 nm (type B), fibres with granules of about 80 to 100 nm (type C) and those with synaptic vesicles of 50 nm (type D) respectively. A fifth very small type contains neither vesicles nor granules. Special attention was paid to synaptic contacts. The divergent dyad seems to be the main type in the frontal ganglion. Frequently, neurosecretory endings are observed in presynaptic position. Immunocytochemical "staining" of neurosecretory material closely corresponds to the distribution of type A fibres, as observed electron microscopically. Immunoelectrophoresis of extracts from frontal ganglia with polyspecific anti-neurosecretion-serum reveals a single precipitation line, indicating that the immunocytochemical localization of neurosecretory material is due to reaction with a specific as well as a cross-reagent anitbody.