The morphology of the Dorsal Unpaired Median (DUM) neurones in the Terminal Abdominal Ganglion (TAG) of the adult male cockroach Periplaneta americana were described based on wholemount preparations and paraffin sections and by using anterograde and retrograde cobalt mapping, octopamine-like immunohistochemistry, and double immunofluorescence technique with both conjugated gamma-aminobutyric acid (GABA) and octopamine antisera. Among 60 +/- 6 neurones with large somata (diameter 40 to 60 microns) on the dorsal midline surface of the TAG that were stained with toluidine blue, about 36 efferent DUM neurones exhibited octopamine-like immunoreactivity. The DUM neurones were arranged in three clusters (anterior, median and posterior) corresponding to the 7th-11th abdominal ganglia of the fused TAG. Anterior efferent DUM neurones with one, two, and four pairs of lateral neurites entered segmental nerves VIIB; VIIB and phallic nerves; IXB and phallic nerves; VIIIA, IXA, X, and IX, respectively. Three octopamine-like immunoreactive DUM neurones innervating heart chambers via segmental nerves (VIIA, VIIIA, and IXA) in the last abdominal segments occurred within abdominal ganglia 7, 8, and 9. Together with octopamine-like immunoreactive efferent DUM neurones, GABA-like immunoreactive dorsal midline neurones with small somata (10 to 20 microns) also occurred within the median group. The spatial distribution of DUM neurones in the TAG suggested that they had their origins in the median neuroblast, as for DUM neurones in the grasshopper.