Different antisera to the molluscan cardioexcitatory peptide FMRFamide, and its fragment, RFamide (Arg-Phe-NH2), label a distinct population of neurons in the optic lobe of the blowfly, Calliphora erythrocephala. Seven morphological types of RFamide/FMRFamide-like immunoreactive neurons could be distinguished in the optic lobes based on the locations of their cell bodies, their axonal projections and the distribution of their processes. Of these, two types could be resolved in their entire extent, the others were labeled only in their cell bodies and terminal processes or were partly obscured by other immunoreactive processes. The RF-like immunoreactive neurons in the optic lobes are of two main classes: (1) two types of large field projection neurons and (2) five types of local neurons. One type of projection neurons (five in each lobe) connects the entire projected retinal mosaic of the medulla and lobula in the optic lobe with protocerebral centres associated with the mushroom body calyx. The other type (2-3 invading each lobe) has cell bodies in the protocerebrum and contralateral processes invading optic lobes. Of the class of local neurons there are two amacrine RF-like immunoreactive neurons in each medulla. Each of these amacrines supplies the entire mosaic with fine processes. The remaining local RF-like immunoreactive neurons are present in relatively large numbers (one type in more than 2000 copies in each medulla) and-supply the medulla, lobula and lobula plate neuropils with fine varicose processes. In the medulla the RF-like immunoreactive processes are arranged in strict layers whereas in the lobula complex the distribution is diffuse. Electron microscopic immunocytochemistry, using both pre-embedding immuno peroxidase-antiperoxidase and post-embedding protein A-gold labeling, was employed for analysis of cytology and synaptic connections of RF-like immunoreactive neurons in the medulla. The varicosities of the processes of the large field projection neurons were not found to make chemical synapses with other neurons in the medulla. The spines of the RF-like immunoreactive processes of the large medulla amacrines, however, make pre- and postsynaptic contacts with other neural elements. Our findings indicate that an RFamide/FMRFamide-like substance may be used as a neurotransmitter or neuromodulator by optic lobe neurons of different types. The local and projection RF-like immunoreactive pathways probably play different roles in visual processing.