The fine structure of the perivascular and the other extraganglionic nerve plexuses in the muscularis externa and the mucosa of the guinea-pig stomach was studied in specimens fixed by perfusion and in specimens fixed by immersion after in vitro incubation in 5-hydroxydopamine. In each of the plexuses, over 50% of the nerves contained ten axons or less; terminal profiles were also much more numerous in sections of nerves of this size than in sections of the larger nerves. Five types of axon terminal were identified in the nerves. Terminals containing mainly large dense-cored vesicles were few, but were more numerous in the perivascular than in the other plexuses. Terminals containing small clear vesicles and large dense-cored vesicles approximately 140 nm in diameter were also few and were confined to the plexuses in the muscularis externa. The remaining types of terminal could be clearly differentiated only in specimens incubated in 5-hydroxydopamine. Terminals containing small clear vesicles were present in large numbers in the plexuses in the muscularis externa and the mucosa. Terminals containing small dense-cored vesicles constituted all but a small proportion of those seen in the perivascular plexuses. They were also seen in small numbers in each of the other plexuses. The remaing type of terminal contained small vesicles which were larger than in the other types and which contained halos of electron-dense material. Such terminals were present only in the plexuses in the muscularis externa and were concentrated in nerves located close to the myenteric plexus.
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