GABA-gated Cl(-) channels were studied in the nervous system of the larval tobacco hawk moth, Manduca sexta, using electrophysiology, (36)Cl(-) uptake into membrane microsacs and immunocytochemistry. A GABA-induced increase in Cl(-) conductance was recorded from a visually identifiable neurone ( fg1) in the desheathed frontal ganglion. The response was insensitive to the vertebrate GABA(A) receptor antagonist, bicuculline, but was blocked by picrotoxinin. Bicuculline-insensitive, picrotoxinin-sensitive, GABA-stimulated (36)Cl(-) uptake was also detected in membrane microsacs prepared from the isolated larval M. sexta nervous system. Such receptors appear to be the major type of GABA receptor in larval nervous system membrane microsac preparations. An antibody raised against a 17 amino acid peptide, based on the predicted C-terminus of the Drosophila GABA receptor subunit (RDL), stained not only cell bodies, including that of fg1, but also the neuropile in the frontal ganglion, indicating the existence of RDL-like GABA receptor subunits in neurones of this ganglion. Thus, bicuculline-insensitive GABA-gated Cl(-) channels are present in the larval nervous system of M. sexta.