Brain-specific binding sites have been isolated on synaptosomal membrane fragments which recognize pharmacologically active benzodiazepines (BDZ's) and triazolopyridazines (TPZ's). While early evidence indicated the existence of a single homogeneous class of BDZ binding sites, more recent biological and pharmacological studies support the notion of BDZ receptor multiplicity. We now propose that two biochemically distinct BDZ receptors exist in brain which are responsible for the mediation of different pharmacological activities. Type I BDZ receptors display a high affinity for both BDZ's and TPZ's, are not coupled to GABA receptors or to chloride ionophores, and are the sites which mediate anxiolytic actions. Type II BDZ receptors display a high affinity for BDZ's, display a low affinity for TPZ's, are coupled to GABA receptors and/or chloride ionophores, and are the sites which mediate pharmacological effects other than anxiolytic activity.