One hundred patients, who were in pain during the immediate postoperative period after upper abdominal operations, were included in this double-blind, between-patient, two-dose study. During N2O-O2-halothane-relaxant anaesthesia no analgesics were given. The patients received 0.07 mg/kg or 0.14 mg/kg of nalbuphine or 0.3 mg/kg or 0.6 mg/kg of pentazocine by intravenous injection. Pain and side effects were assessed for 4 h after administration of the test drug, or until the pain returned to the pre-injection level, when a conventional analgesic was given. The onset of pain relief was similar and the peak effect occurred about half an hour after the injection after both drugs. On a milligram basis, nalbuphine seemed to be about three times as potent as pentazocine. The duration of action seemed to be slightly longer after nalbuphine, but 2 1/2 hrs. after the injection the pain had returned to preinjection level in 2/3 of the patients, even after the higher doses of both drugs. Except for sleepiness, there were few side effects and they were similar after both drugs. No psychotomimetic effects were observed.