The aim of this study was to describe the architecture of a ganglionated nerve plexus found in the loose connective tissue surrounding the pulmonary vein of the mouse. The input to this plexus was from the vagus nerves and from the stellate ganglia. A large ganglion containing more than 200 neurons was commonly found near the primary bifurcation of the pulmonary vein. The neurons were studied by NADH-diaphorase, zinc iodide-osmium and glyoxylic acid-induced catecholamine fluorescence methods at the light microscopic level, by scanning electron microscopy after the removal of connective tissue, and by transmission electron microscopy. The shape of the neuronal cell bodies was generally a smooth ellipsoid with the average major axis about twice the minor axis. The measured maximum cell diameter ranged from 14 to 42 micron (mean 26 micron). The profile area of individual neurons, as measured from wholemount preparations, ranged from 100 to 800 micron2 (mean 340 micron2) and the calculated neuronal volume ranged from 500 to 12,000 micron3 (mean 3300 micron3). Although there was this wide spread in neuronal size, histograms of cell size showed no separate populations of neurons. Almost all of the ganglionic neuronal cell bodies showed no catecholamine-specific fluorescence, but about 1% of the neurons exhibited a weak green fluorescence. Only a few noradrenergic nerve fibres were seen within the ganglia and these were associated with intraganglionic blood vessels. Small, intensely fluorescent cells were only rarely associated with the ganglia. Neurons and satellite cells formed units which were surrounded by an intraganglionic connective tissue space and a perineurium. Some of the intraganglionic capillaries were fenestrated. Neurons were entirely surrounded by satellite cells and did not appear to have any long dendrites. The generally smooth neuronal cell bodies had short spine-like processes, which were confined to within the satellite cell sheath. Preganglionic nerve fibres formed pericellular baskets of varicose fibres around neurons and made synapses either directly on the cell body or on somatic spines in about equal numbers. No synapses were found in the neuropil at a distance from the neuronal cell body. A few nerve processes were deeply embedded within the neuronal cell body. Clusters of vesicles were found in the cytoplasm of most neurons and were associated with subplasmalemmal densities. These synapse-like structures were mostly directed towards satellite cells, but some were associated with incoming synapses.