(+)-Haedoxan A, an insecticidal sesquilignan, inhibited the specific binding of the noncompetitive GABA antagonist [S-35]t- butylbicyclophosphorothionate (TBPS) to rat brain membranes in concentration-dependent and noncompetitive manners. The data show haedoxan's interaction with a site coupled to the TBPS-binding site on the GABA(A) receptor in rat brain, while the physiological significance of the interaction remains to be discovered.