Benzodiazepines are widely used as anxiolytics, sedatives, muscle relaxants, and anticonvulsants. They allosterically modulate GABA type A (GABA(A)) receptors by increasing the apparent affinity of the agonist GABA to elicit chloride currents. Such an increase in apparent affinity of channel gating could either be caused by an increase in affinity for GABA or by a facilitation of channel opening. In the first case, conformation of the affected sites would have to be altered. In the second case, the affected sites are not necessarily altered, because diazepam could facilitate conformational changes leading to the open channel. It is controversial as to whether benzodiazepines affect only channel opening induced by the occupation of one of the two agonist binding sites or by both. We used receptors formed by concatenated subunits to selectively destroy one of the two agonist sites by point mutation. Both of the receptors harboring only one active agonist site could be stimulated by diazepam. We therefore present evidence that binding of diazepam can affect channel opening induced by either agonist binding site.