In addition to anxiolytic and anticonvulsant properties, benzodiazepines (BDZ) produce sedation, ataxia, and muscular relaxation. In general, it was difficult to separate these properties within this chemical class during the search for clinically useful anxiolytics; and when BDZ's were used to characterize 3H-BDZ binding sites they indicated only a single homogenous class of receptors. A new chemical series was discovered, triazolopyridazines (TPZ, prototype CL 218,872), which showed anticonflict activity in rats and monkeys without sedation or ataxia and inhibited 3H-BDZ binding in brain membranes with kinetic characteristics suggesting the presence of multiple BDZ receptors. High affinity and low affinity sites for the TPZ were demonstrated, the former designated at Type 1 and the latter as Type 2. Anatomical and in vivo studies have supported different distributions of each receptor in brain. Lately, the physical separation of discrete proteins which bind 3H-BDZ has been reported. The multiple receptors and the variety of endogenous substances which have been proposed as modulators and ligands of the receptors might explain variability as well as selectivity in pharmacological properties in these drugs.