The imidazopyridine zolpidem has previously been shown to displace benzodiazepines from their receptors, with a preferential activity at BZ1 sites, and to exert hypnotic activity in man. Zolpidem's pharmacological profile includes anticonvulsant, antipunishment and behavioral depressant actions. However, unlike benzodiazepines, zolpidem exerts sedative actions at relatively low doses. In drug discrimination experiments differences between the internal stimuli produced by chlordiazepoxide and zolpidem were identified. These differences appeared to be qualitative rather than quantitative with the stimulus properties of zolpidem being related to the drug's sedative action. In condition where tolerance developed rapidly to the depressant activity of benzodiazepines little tolerance was seen with zolpidem. The behavioral profile of consistent with the suggestion that this compound may have selective sedative action produced by activity at a sub-type of benzodiazepine receptor.