An antiserum against the cockroach cardioactive peptide corazonin was used to investigate the distribution of immunoreactive neurons and neurosecretory cells in the nervous system of the blowfly, Phormia terraenovae, during postembryonic development. A small number of corazonin-immunoreactive neurons was found at larval, pupal, and adult stages. At all postembryonic stages two cell groups were found in the protocerebrum of the brain: 1) two lateral cell clusters and 2) two median cells. In the larva eight bilateral cell pairs were found in thoracic and abdominal neuromeres of the fused ventral ganglion. The lateral brain neurons are located in the lateral neurosecretory cell group and extend axons with branches in several components of the retrocerebral neuroendocrine complex, in the stomatogastric nervous system of larvae and adults, and additionally in muscles of the alimentary canal in the adult. The most prominent element of these peripheral processes is a large plexus of varicose fibers located in the wall of the aorta, the main site for the release of neurohormones produced in the brain of blowflies. The presence of corazonin-immunoreactive material in the aortic plexus suggests that this peptide functions as a neurohormone. During metamorphosis, the immunoreactive neurons found in the thoracic-abdominal ganglion of the larva disappear, and in the brain new immunoreactive neurons are added to those that persist from larval stages. The bulk of the corazonin-immunoreactive material extracted from adult brains and corpora cardiaca-aorta complexes was found to co-elute with synthetic corazonin in reversed-phase high-performance liquid chromatography as monitored with enzyme-linked immunosorbent assay.