Pelvic ganglia contain a mixture of sympathetic and parasympathetic neurons which are innervated by either lumbar (sympathetic) or sacral (parasympathetic) preganglionic axons, respectively. However, until recently no stain for these axon terminals has been available. In the present study of the male rat major pelvic ganglion, a ganglion which supplies axons to the lower urinary and digestive tracts and internal reproductive organs, the total population of preganglionic axon terminals was immunostained using an antiserum against synaptophysin, a protein associated with all small synaptic vesicles (such as the presumptive cholinergic vesicles present in all preganglionic terminals). Selective bilateral lesions of either the hypogastric or pelvic nerves, which carry the sympathetic and parasympathetic preganglionic axons, respectively, were carried out and three to seven days later ganglia were examined immunohistochemically for the distribution of residual synaptophysin-positive terminals. Neurons remaining innervated following hypogastric nerve lesion were therefore classified as parasympathetic and those innervated after pelvic nerve section were defined as sympathetic. These two cell groups are present in approximately equal proportions. Double-staining immunofluorescence to identify which transmitters or peptides are present in either sympathetic or parasympathetic neurons showed that the majority (approximately 75%) of sympathetic neurons are presumed to be noradrenergic (i.e. contain tyrosine hydroxylase immunoreactivity) and are also immunoreactive for neuropeptide Y; the remainder contain vasoactive intestinal peptide-immunoreactivity but not tyrosine hydroxylase and may be cholinergic. Parasympathetic neurons were virtually all non-noradrenergic (tyrosine hydroxylase negative) and were also of two histochemical types, with some neurons containing neuropeptide Y- and others containing vasoactive intestinal peptide-immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)