The Leu-callatostatins are a series of four neuropeptides isolated from nervous tissues of the blowfly Calliphora vomitoria that show C-terminal sequence homology to the allatostatins of cockroaches. The allatostatins have an important role in the reproductive processes of insects as inhibitors of the synthesis and release of juvenile hormone from the corpus allatum. In this study, the distribution of the Leu-callatostatin-immunoreactive neurones and endocrine cells has been mapped in C. vomitoria and, in contrast to the cockroach allatostatins, it has been shown that there is no cytological basis to suggest that the dipteran peptides act as regulators of juvenile hormone. Although occurring in various neurones in the brain and thoracico-abdominal ganglion, there is no evidence of Leu-callatostatin-immunoreactive pathways linking the brain to the corpus allatum, or of immunoreactive terminals in this gland. Three different types of functions for the Leu-callatostatins are suggested by the occurrence of immunoreactive material in cells and by the pathways that have been identified. (1) A role in neurotransmission or neuromodulation appears evident from immunoreactive neurones in the medulla of the optic lobes, and from immunoreactive material in the central body and in descending interneurones in the suboesophageal ganglion that project to the neuropile of the thoracico-abdominal ganglion. (2) Leu-callatostatin neurones directly innervate muscles of the hindgut and the heart. Immunoreactive fibres from neurones of the abdominal ganglion pass by way of the median abdominal nerve to ramify extensively over several areas of the hindgut. Physiological experiments with synthetic peptides show that the Leu-callatostatins are potent inhibitors of peristaltic movements of the ileum. Leu-callatostatin 3 is active at 10(-16) to 10(-13) M. This form of regulatory control over gut motility appears to be highly specific since the patterns of contraction in other regions are unaffected by these peptides. (3) Evidence that the Leu-callatostatins act as neurohormones comes from the presence of varicosities in axons passing through the corpus cardiacum (but not the corpus allatum) and also from material in extraganglionic neurosecretory cells in the thorax. Fibres from these peripheral neurones are especially prominent over the large nerve bundles supplying the legs. There are also a considerable number of Leu-callatostatin-immunoreactive endocrine cells in a specific region of the midgut. The conclusion from this study is that although conservation of the structure of the allatostatin-type of peptides is evident through a long period of evolution it cannot be assumed that all of their functions have also been conserved.(ABSTRACT TRUNCATED AT 400 WORDS)