The pentapeptide proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH) is a well-studied bioactive substance in insects. With an antiserum against proctolin we have mapped proctolinlike-immunoreactive (PLI) neurons in the nervous system of the blowfly Calliphora erythrocephala. In the brain, including the suboesophageal ganglia, 80-90 neurons were found to be PLI. A further 200-250 PLI neurons innervate the lobula of the optic lobe. The thoracic ganglia contain 100-130, and the abdominal ca. 60 PLI neurons. In the brain and ventral ganglia the immunoreactive neurons are of different types: interneurons, efferents (possibly some motorneurons), and neurosecretory cells. Some of these neurons are individually identifiable; others can be identified collectively as clusters. Identifiable neurons innervate protocerebral neuropil associated with the pars intercerebralis and the beta-lobes of the mushroom bodies as well as tritocerebral neuropil. Some of the prominent clusters innervate the central body of the protocerebrum, tritocerebrum, and possibly leg motor neurons. One abdominal cluster is of special interest because it consist of efferent neurons with processes in the lateral abdominal nerves. Some of these processes are located in the neural sheath in neurohaemal regions, and electron microscopy demonstrates that their terminals are outside the blood-brain barrier. The PLI processes in the protocerebrum contain large granular vesicles and form chemical synapses with different kinds of nonimmunoreactive neural elements. Thus, in Calliphora the proctolinlike substance may be used as a central transmitter/modulator, a neuromuscular transmitter, and a neurohormone released into the circulation.