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Cage convulsant t-butyl bicyclophosphoro[3sS]thionate binding activity in rat brain membrane homogenates was solubilized with the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]propane sulfonate (Chaps) and shown to co-purify with the benzodiazepine -y-aminobutyric acid (GABA) receptor complex on gel filtration and affinity chromatography. Whereas convulsant binding activity, but not GABA and benzodiazepine receptor binding, was eliminated by solubilization in other detergents like sodium deoxycholate or Triton X-1 00, or by addition of Triton X-100 to the extracts solubilized in the zwitterionic detergent, convulsant activity was not irreversibly lost or selectively unstable, but could be restored by exchanging the protein back into the detergent Chaps. The GABA -benzodiazepine receptor activity solubilized in Chaps alone, containing convulsant activity, and a sample in Chaps supplemented with Triton X-100 and lacking convulsant activity, did not differ in size as measured by gel filtration column chromatography or by radiation inactivation target size analysis. This suggests that convulsant binding activity does not require any additional protein subunits or other macromolecules nor any unique aggregation state relative to GABA and benzodiazepine receptor binding, and that all three activities reside on the same protein complex. As in intact brain, the target size for convulsant binding activity was 3 -5 times that of benzodiazepine binding activity, suggesting that an oligomeric protein structure of the receptor complex with intact strong subunit interactions present in the native membrane environment is needed for convulsant activity, and that this and other properties are more preserved in Chaps than in other detergents.Neuronal membrane chloride channels activated by the major inhibitory neurotransmitter, y-aminobutyric acid (GABA), are inhibited by convulsant drugs such as the naturally occurring picrotoxinin [l -31. This inhibitory action is mediated through the binding of convulsants to a receptor site that is part of an oligomeric protein complex consisting of GABA receptor sites, benzodiazepine receptor sites, and barbiturate receptor sites coupled to a chloride channel [3 -71. Binding sites for these convulsant drugs have been assayed in vitro with the radioactive picrotoxin analog, [ 3 H ]~-dihydropicrotoxinin [8,9] propane sulfonate (Chaps), and to co-purify with the GABA -benzodiazepine receptor complex [17,24]. Likewise, GABA enhancement of benzodiazepine binding is preserved to a greater extent in Chaps-solubilized preparations than in other detergents [25].Pretreatment of membranes with Triton X-100 causes perturbation of the GABA -benzodiazepine receptor complex which leads to an increase in the affinity of [3H]