Using single and double staining immunohistochemical procedures it has been demonstrated that an avian pancreatic polypeptide-like immunoreactivity (APP-LI) co-exists with catecholamines in neurones within the A1/A3, A2 and A6 (locus coeruleus) cell groups, and also with methionine-enkephalin in cell bodies of the sacral parasympathetic system. Axonal and terminal immunoreactivity was observed in axons and nerve terminals in particularly high density within the dorsal horn of the spinal cord and originated in part from intrinsic cell bodies. Intense terminal labelling was also found around sacral and caudal lumbar motor neurones, and thoracic lateral sympathetic column neurones. Immunoreactive material was present in fibres around neurones of the periaqueductal grey, caudal raphe and the locus coeruleus and in neurones in the arcuate nucleus.