The quantitative cell count showed the presence of about 20,000 ganglion cells with associated satellite elements in the nodose ganglion in the monkey. Among these closely packed cells, at least one-third were substance P (SP) immunoreactive, ranging from faint or moderate to intense staining. Substance P immunoreactivity (SP-IR) was localised in the cell bodies and their long extending neurites. Ultrastructural study showed that SP-IR was not associated with any particular organelles or inclusions. A striking feature of the nodose ganglion was the occurrence of SP-positive pericellular arborisations associated with about 0.5% of the ganglion cells which were almost exclusively SP-negative. The pericellular arborisation displayed diverse morphological forms from a simple tortuous fibre to complex glomerular networks or plexuses encircling the soma of SP-negative ganglion cells. The varicose nerve fibres forming the pericellular arborisations appeared to terminate as 'boutons' contacting the soma of the ganglion cells. Electron microscopic study demonstrated the close spatial relation between the SP-IR profiles and the ganglion cell but there was no direct synaptic contact. In some instances, the SP-IR profiles containing agranular and dense-cored vesicles penetrated the cytoplasm of satellite cells, almost reaching the surface of the soma of the ganglion cell. The sources of origin of the nerve plexuses in the pericellular arborisation were either from the small and sparsely distributed jugular ganglion cells which were intensely SP-IR or from the intrinsic SP-IR nodose ganglion cells. The possibility that the efferent neurons in the dorsal motor nucleus of the vagus could also contribute to the pericellular arborisation was also considered. The function of the pericellular arborisations may be related to the modulation of the SP-deficient ganglion cells with which they associate through the release of SP and probably by way of the satellite cells.