Autonomic ganglia in the chicken pancreas have been electron-microscopically investigated. They were composed of ganglion cells existing singly or making small groups and bundles of mainly unmyelinated nerve fibers.<br>1. The perikaryonal cytoplasm of chicken intrapancreatic ganglion cells contains large granular vesicles 1, 000-1, 500Å in diameter, dense bodies (probably lysosomes) and occasionally multivesicular bodies. The Nissl substance consisted of cisternae of endoplasmic reticulum studded by relatively sparse ribosomes and abundant free ribosomes. The nucleus of the ganglion cells located eccentrically in the perikaryon occasionally contained rod-shaped inclusion bodies consisting of a bundle of parallel filaments or fibrils.<br>2. The ganglion cells project their dendrites in a multipolar form, but a neurite could not be identified. The dendrites are electron dense in appearance and contain ample Nissl substance associated with free ribosomes. In addition, they contain all the organelles which could be found in the perikaryon.<br>3. Unmyelinated preganglionic nerve fibers make large nerve bundles mixed with a few thin myelinated nerve fibers. Their axoplasm contains large granular vesicles identical with those found in the perikarya of intrapancreatic neurons, small agranular vesicles about 400-800Å in diameter, mitochondria and occasionally dense bodies.<br>4. Many axo-dendritic synapses were demonstrated, but the presence of axosomatic synapse could not be proved. The existence of axo-axonal synapses was doubtful. The axo-dendritic synapses are usually found near the perikarya; the widened synaptic terminals of preganglionic fibers occur near the base of the dendrites.<br>5. In the axo-dendritic synapses both pre- and post-synaptic membranes are slightly thickened and show a conspicuous density increase, and a dense material is condensed beneath both membranes. The synaptic cleft (about 300Å in width) is filled with a finely granular dense material. A “bi-focal” synaptic area, i.e., a synaptic area divided into two smaller areas by an interruption of the membrane specializa tion, is occasionally observed.<br>6. Axo-dendritic synapses contain two types of synaptic vesicles: abundant small round or oval agranular vesicles 400-800Å in diameter and less numerous large granular vesicles 1, 000-1, 500Å. The latter are identical with those found in perikarya and preganglionic axons. In the majority of the synaptic terminals there occur elongated or flattened vesicles in varying numbers, intermingling with the small round agranular vesicles.<br>7. The satellite cells, characterized by the high density of their cytoplasm, invest the perikaryon, dendrites and preganglionic axons with their thin cytoplasmic extensions. At interruptions of this investment the basement membrane directly covers the naked surface areas of the neuronal elements. To the contrary, preganglionic axons are sometimes wrapped by a double or triple-layered cytoplasmic sheet of peripheral satellite cells, like the so-called “tunicated nerve fibers.”