Light and electron-microscope observations have been made on the enteric nervous system of the chick and pigeon gizzard. In both species, Auerbach's plexus is in the form of a condensed, ganglionated nerve trunk, which is plexiform in certain regions. Since the outer longitudinal muscle coat of the gizzard is lost during development, the mam body of Auerbach's plexus lies immediately below the serosa; frequently there are extensions from the plexus into the underlying musculature. The ganglion cells and the associated nerve fibres that derive from these extensions are scattered throughout the lamina muscularis. Large numbers of myelinated axons are present in Auerbach's plexus. In the chick, ganglion cells occur predominantly at the nodes of the plexus, but in the pigeon they occur apparently randomly along the nerve tracts. There are synapses between unmyelinated fibres and the ganglion cells, either on the cell soma or on processes from it. All synapses are characterised by a wide (250–300 A) cleft; presynaptic specialisations are resolvable into irregular densities which project into the cytoplasm. Presynaptic axon profiles contain either agranular vesicles only, or both agranular and large granular vesicles, together with occasional small granular vesicles. However, in all cases, the vesicles immediately associated with synaptic areas of the membrane are small and agranular. Many preganglionic axon profiles, lying close to the somata of ganglion cells, contain granular vesicles alone. Such nerve fibres do not form specialised synapses with ganglion cells. A nerve plexus comparable to Meissner's plexus of mammals is absent.