The analgesic efficiency of ketamine and pethidine was compared in experimental ischemic pain and postoperative pain after oral surgery. Naloxone 1.6 mg or placebo was given 5 min before the analgesic drug. The subjects recorded their pain on a visual analogue scale. Both ketamine 0.3 mg/kg and pethidine 0.7 mg/kg were effective as analgesics against the two types of pain studied. Naloxone prevented the analgesic effect of pethidine, but had no effect on ketamine analgesia. The results are in accordance with the hypothesis that the analgesic effect of ketamine is mediated by a non-opioid mechanism, possibly involving PCP-receptor-mediated blockade of the NMDA-receptor-operated ion channel.