The CNS of the cockroach Periplaneta americana contains saturable, specific binding sites for [(3)H]GABA, [(3)H]flunitrazepam and [(35)S]TBPS. The [(3)H]GABA binding site exhibits a pharmacological profile distinct from that reported for mammalian GABA(A) and GABA(B) receptors. The most potent inhibitors of [(3)H]GABA binding were GABA and muscimol, whereas isoguvacine, thiomuscimol and 3-aminopropane sulphonic acid were less effective. Bicuculline methiodide and baclofen were ineffective. Binding of [(35)S]TBPS was partially inhibited by 1.0 x 10(?6) M GABA, whilst binding of [(3)H]flunitrazepam was enhanced by 1.0 x 10(?7) M GABA. The pharmacological profile of the [(3)H]flunitrazepam binding site showed some similarities with the peripheral benzodiazepine binding sites of vertebrates, with Ro-5-4864 being a far more effective inhibitor of binding than clonazepam. Thus a class of GABA receptors with pharmacological properties distinct from mammalian GABA receptor subtypes is present in insect CNS.