Morphological and electrical properties of neurons with somata in the pars intercerebralis (PI) and pars lateralis (PL) were examined by intracellular recording and staining in the adult blow fly, Protophormia terraenovae. According to the location of somata and fiber distribution, two types of PI neurons (PIa and PIb) and two types of PL neurons (PLa and PLb) were identified. PIb neurons were further divided into two subgroups of PIb1 and PIb2 depending on fiber branching patterns in the retrocerebral complex. PIa neurons projected axons to the contralateral nervi corporis cardiaci, whereas PLa and PLb neurons projected axons to the ipsilateral nervi corporis cardiaci. PIb neurons characteristically showed symmetrical morphology with their somata along the midline. PLb neurons had a large branching area in the subesophageal ganglion. In the retrocerebral complex, PIb2 and PLa neurons sent fibers into the corpus allatum. PIa, PIb1 and PLb neurons projected not to the corpus allatum but to the corpus cardiacum-hypocerebral complex or visceral muscles in their vicinity. PIa, PIb and PLa neurons showed long spike durations (3-10 ms). PLb neurons were immunoreactive with antisera against corazonin, FMRFamide, or beta-pigment-dispersing hormone. This is the first report revealing the morphology of individual neurons with somata residing in PI and PL in the adult fly.