Neuropeptide-like immunoreactivity to antisera raised against Leu- and Met-enkephalin, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY) and substance P (SP) have been studied immunohistochemically in middle cervical and stellate ganglia of dogs. To investigate the relationship of the peptides to one another as well as to preganglionic and postganglionic neurons, intact and chronically decentralized middle cervical and stellate ganglia were studied. Ganglia were processed for immunohistochemistry in unoperated dogs and in dogs two weeks after unilateral ganglionic decentralization. The immunoreactivity for each peptide had a characteristic distribution in the ganglia. These distributions differed from one another and from the distribution of cardiac postganglionic sympathetic neurons. Camera lucida drawings of peptide distributions were made to compare different peptides and counts were made to determine the percentages of cells immunoreactive for a given peptide. The results demonstrated that enkephalin-like immunoreactivity in axons was present in both the stellate and middle cervical ganglia, but was heaviest in the caudal 2/3 of the stellate ganglia. Enkephalin-like immunoreactive fibers formed pericellular baskets around stellate ganglion neurons. VIP-like immunoreactive cell bodies and processes were distributed sparsely, but widely, in the stellate ganglia and to a lesser extent in the middle cervical ganglia. One of two commercial antisera to SP resulted in immunoreactive staining of cell bodies and processes in the stellate ganglia. SP-like immunoreactivity in neurons represented about 10% or less of the cells in the stellate ganglia. At least 80-85% of the neurons in the stellate and middle cervical ganglia were immunoreactive for NPY antisera. Decentralization eliminated enkephalin-like immunoreactive staining in the middle cervical and stellate ganglia, but not the VIP-, NPY- and SP-like immunoreactive staining of neurons in these ganglia. In summary, the enkephalin-like immunoreactive axons in the thoracic autonomic ganglia appear to be derived from extrinsic neurons, most likely from preganglionic spinal neurons. VIP-, SP- and NPY-like immunoreactivity were not significantly affected by decentralization. The results provide anatomical evidence for substrates related to neuropeptidergic synaptic mechanisms in thoracic autonomic ganglia.