A polyclonal antibody raised against a 17 amino acid polypeptide (the predicted C-terminal sequence of the cloned Drosophila melanogaster gamma-aminobutyric acid (GABA) receptor subunit, RDL) was used to investigate the distribution of GABA receptor subunit(s) of this type in the nervous system of the cockroach Periplaneta americana. Intense staining was detected in the calyces of the mushroom bodies, glomeruli of the antennal lobes, lower central body, the corpora cardiaca and several cell layers of the medulla and the lobula regions of the optic lobe. The most intense immunocytochemical staining was in the suboesophageal ganglion. Control sections pre-incubated with the primary antibody and conjugated peptide were not stained. Thus, it appears that a GABA receptor subunit of the RDL type is located in cockroach brain regions involved in the processing of visual, olfactory and mechanosensory inputs to the nervous system. Since the corpora cardiaca reacted to this antiserum, this type of GABA receptor may also be involved in the regulation of neurosecretory activity.