Alprazolam is a triazolobenzodiazepine, with a potent anxiolytic action and a short half-life. Alprazolam analgesia was measured, using the radiant heat tailflick assay in mice, which was administered alone or in combination with opioids. Intrathecally administered alprazolam produced a dose-response increase in the tailflick latency with an ED50 of 34 microg (19.4-72.5, 95% CL). There were almost no effects after intracerebroventricular injections. Naloxone almost completely abolished the analgesia response mediated by alprazolam. This sensitivity to naloxone indicates that at least some of the analgesic effects of alprazolam are mediated by an opioid mechanism of action. When administered together with various antagonists of opioid receptor subtypes, we found that the mu antagonists, but not the delta and kappa1 subtypes inhibited alprazolam analgesia significantly. No effect was found when alprazolam was coadministrated with kappa3 opioid agonists. In addition, we found a supra-additivity (synergistic) increase in analgesia when alprazolam was given with morphine. Competition binding assays show the highest affinity of alprazolam to the mu1 subtype. In summary, we conclude that alprazolam mediates its analgesic effect, most probably via an mu opiate mechanism of action.