The actions of the polychlorocycloalkane insecticide heptachlor, and its epoxide metabolite, were examined on GABA receptors in insects and vertebrates. Electrophysiological experiments on the cell body of the cockroach (Periplaneta americana) fast coxal depressor motor neuron (Df), and GABA-activated 36Cl- uptake experiments on microsacs prepared from cockroach ventral nerve cords showed that both heptachlor and heptachlor epoxide blocked functional GABA receptors. The block appeared to be non-competitive and was voltage-independent over the membrane potential range -75 mV to -110 mV. There was no significant difference between the potencies of heptachlor and heptachlor epoxide in the functional assays for insect GABA receptors. Both compounds inhibited [35S]-t-butylbicyclophosphorothionate [( 35S]TBPS) binding in insects and vertebrates. The findings provide further evidence for block of an insect GABA receptor/Cl- channel by the cyclodiene class of polychlorocycloalkanes, and reveal differences in the insecticide-[35S]TBPS binding site interactions of insects and vertebrates.