The "cage" convulsant [S-35]t-butylbicyclophosphorothionate(S-35]TBPS) binds with high affinity to specific sites "at" or "near" a gamma-amino-butyric acid (GABA)-gated chloride channel according to current hypothesis. We now report that pretreatment of membranes with 2,3-butanedione, an arginine-specific reagent, causes a dose- and time-dependent decrease in the number of [S-35]TBPS binding sites. No decrease occurs when membranes are pretreated with 2,3-butanedione in the presence of picrotoxinin. Binding of [S-35]TBPS to the remaining sites occurs with the same characteristics as binding to the untreated receptor population.