A previously identified irreversible affinity label for the gamma-aminobutyric acid (GABA) binding site in rat brain membranes, m-sulphonate benzene diazonium chloride (MSBD), was characterized in functional studies using patch clamp and two-electrode voltage clamp recording techniques. MSBD did not exhibit any agonist activity on native GABAA receptors in cultured sympathetic ganglionic neurones but acted as an antagonist of GABA-induced membrane currents. Recombinant GABAA receptors composed of alpha 1, beta 1 and gamma 2S subunits were expressed in Xenopus oocytes following microinjection with cDNAs. Equilibrium dose-response curve analyses established that MSBD was a partially reversible, apparently non-competitive GABAA receptor antagonist. The IC50 for MSBD was estimated from an inhibition curve as 87 +/- 3 microM. In addition, the onset and recovery from MSBD-induced inhibition was independent of GABAA receptor activation. The relatively simple structure of this novel GABAA receptor antagonist, MSBD, is compared with known agonists and antagonists at the GABAA receptor. MSBD may be a useful pharmacological tool which could be used to deduce further information about the structure and function of agonist and antagonist binding sites on the GABAA receptor.