The distribution of a neuropeptide, previously shown to have the same or a very similar amino acid composition as vertebrae pancreatic polypeptide (PP), has been studied in the nervous system gut of the blowfly, Calliphora vomitoria. Neurones immunoreactive to a bovine PP antiserum occur in the thoracic and abdominal ganglionic components of the central nervous system, in addition to the brain and suboesophageal ganglion. Pancreatic polypeptide appears to be relayed from its cells of origin to a neurohaemal organ in the dorsal sheath of the thoracic ganglion. PP immunoreactivity is also found in cells of the hypocerebral ganglion of the stomatogastric nervous system and in associated nerve fibres. The mid-gut contains PP-positive material in flask-shaped cells of its epithelial lining.