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1. Many of the nerve cells comprising the cardiac parasympathetic ganglion of the mudpuppy are spread out in a thin, transparent sheet of tissue, enabling one to see cellular details in living preparations with differential interference contrast optics. The aim of this study was twofold: to establish the morphology of the nerve cells and their synaptic connections by light and electron microscopy, and to determine which aspects of the ganglion's structure could be reliably identified in the living tissue. 2. There are two types of neurones in the ganglion: (a) principal cells that send post-ganglionic axons to cardiac muscle fibres, and (b) interneurones whose processes are confined to the ganglion. 3. Interneurones are distinguished from principal cells by the presence of numerous granular vesicles seen with the electron microscope, and by intense formaldehyde-induced fluorescence. The interneurones are thus similar to catecholamine-containing interneurones in autonomic ganglia of other vertebrates. 4. Principal cells are innervated by processes that terminate mainly on the cell body, forming up to forty-five synaptic boutons and covering, on the average, 5% of the perikaryal surface. The synaptic terminals are derived from three sources: (a) axons from the vagus nerves, (b) interneurones and (c) other principal cells. Vagal terminals contacting principal cells contain agranular vesicles typical of preganglionic cholinergic endings. At regions of contact between processes of interneurones and principal cells, the interneurones have granular vesicles focused at membrane specializations; in addition there are small areas of close plasma membrane apposition, probably gap junctions. Some of the contacts between principal cells are characterized by gap junctions; others are structurally similar to vagal endings but persist after vagal degeneration. 6. Interneurones are innervated by axons that make contact mainly with their processes. The axon terminals on processes of interneurones contain agranular vesicles similar to vagal terminals on principal cells. 7. In live preparations principal cells are distinguished from interneurones by their size and the appearance of their organelles. Synaptic contacts on principal cells could often be identified and, in some cases, large contacts from interneurones or those from other nearby principal cells could be traced back to their cell bodies of origin. The validity of these identifications was confirmed by subsequent electron microscopic examination of the same cells.