Peptide-FMRFamide-like immunoreactivity in the brain and suboesophageal ganglion of the honeybee Apis mellifera L. is demonstrated with the peroxidase-antiperoxidase technique. Immunoreactivity is found in about 120 perikarya of the brain and in about 30 of the suboesophageal ganglion. These cells are distributed in 13 paired clusters representing neurons of different types including neurosecretory neurons projecting to neurohemal organs. Immunoreactivity of different intensity is found in the non-glomerular neuropil around the mushroom bodies, in the lateral protocerebrum, the central body, the optic tubercles, the lobula and medulla of optic lobe, the ocellar neuropil, in multiglomerular elements of the antennal lobes and in the dorsal deuterocerebrum. In the mushroom bodies, immunoreactivity is located in layers of the lobes and stalks, corresponding to intrinsic fibre bundles of some Kenyon cell types. The somata of these intrinsic cells did not show FMRFamide-like immunoreactivity. Electron microscopy of immunostained somata and nerve fibres was performed employing a pre-embedding peroxidase-antiperoxidase technique. Fibres of optic lobes and the non-glomerular neuropil contain immunoreactive dense core vesicles (diameter 50-165 nm) accumulated in boutons besides small synaptic vesicles and synaptic membrane specializations. Immunoreactive layers of the mushroom body neuropil were analysed at the ultrastructural level. Axon profiles with dense-core vesicles of a small type (diameter 35-75 nm) show only faint immunoreactive products. Immunoreactivity of intrinsic mushroom body neurons does not appear to be specifically correlated with synaptic organelles. Our results indicate that FMRFamide or related peptides peptides may be neuroactive compounds in different classes of nerve cells in the bee brain.