This study demonstrates the localization and regulation of a novel neuropeptide of 33 amino acids, secretoneurin (SN), in the rat superior cervical ganglion. Gel filtration chromatography of ganglion proteins followed by a specific radioimmunoassay revealed that SN is the predominant cleavage product of secretogranin II, a member of the chromogranin/secretogranin protein family, in adult ganglia. SN was detected within the majority of nerve endings surrounding postganglionic neurons that were identified by the presence of synaptophysin and, in part, colocalized leu-encephalin. Applying immuno-electronmicroscopy, SN was localized to large dense core vesicles of neuronal and small intensely fluorescent (SIF) cells. In situ hybridization revealed the presence of secretogranin II mRNA in postganglionic neurons and, to a lesser extent, in SIF cells. One week after transection of the postganglionic branches SN levels were not significantly altered; however, a decrease of secretogranin II mRNA was observed in postganglionic neurons but not in SIF cells. After decentralization of the ganglion, SN-immunoreactive nerve terminals disappeared and intraganglionic SN levels were reduced by 70%, indicating the preganglionic origin of SN-positive nerve fibres and varicosities. Secretogranin II mRNA was slightly reduced under this condition. Combined axotomy and decentralization further diminished intraganglionic secretogranin II mRNA, although peptide levels increased significantly above control values under these conditions. Double-labelling immunofluorescence with antibodies against the somatodendritic marker microtubule-associated protein 2 (MAP2) revealed that the increase in SN immunoreactivity was due to an accumulation of SN in axonal processes of postganglionic neurons. SN immunoreactivity was also detected in dissociated neonatal superior cervical ganglion cultures and increased significantly upon treatment with nerve growth factor, the survival and differentiation factor of sympathetic neurons during perinatal development. Co-culture with non-neuronal cells or addition of leukaemia inhibitory factor, a cytokine known to stimulate synthesis of various peptides after nerve transection, did not influence SN immunoreactivity. Therefore, since no fixed relationship between SN and any of the known neuropeptides or neurotransmitters expressed in sympathetic neurons was observed, the expression of this novel peptide appears to be independently regulated.