The effect of naloxone on dental postoperative pain was studied to examine the hypothesis that endorphins mediate placebo analgesia. All patients had extraction of impacted mandibular third molars with diazepam, N₂O, and local block with mepivacaine. 3 h and 4 h after surgery naloxone or a placebo was given under randomised, double-blind conditions. Pain was evaluated on a visual analogue scale. Patients given naloxone reported significantly greater pain than those given placebo. Patients given placebo as their first drug were either placebo responders, whose pain was reduced or unchanged, or nonresponders whose pain increased. Naloxone given as a second drug produced no additional increase in pain levels in nonresponders but did increase pain levels of placebo responders. Nonresponders had a final mean pain rating identical to that of responders who received naloxone as their second drug. Thus the enhancement of reported pain produced by naloxone can be entirely accounted for by its effect on placebo responders. These data are consistent with the hypothesis that endorphin release mediates placebo analgesia for dental postoperative pain.