In a double-blind clinical trial of 48 patients, nalbuphine, morphine, and pethidine were compared by on-demand intravenous analgesia during the first 24 hours after cholecystectomy. Overall pain relief (visual analogue score) was recorded by the patients as 50 (SEM 4) for nalbuphine, 44 (SEM 4) for morphine and 53 (SEM 5) for pethidine. These scores were not significantly different. The mean demand for each drug over the 24-hour period was 70 (SEM 12) mg for nalbuphine, 46 (SEM 6) mg for morphine and 614 (SEM 49) mg for pethidine. Pain on movement, either during deep breathing or turning, was found to be less well controlled after nalbuphine (70, SEM 2), and pethidine (67 SEM 7) than after morphine (52, SEM 5; p less than 0.01). The incidence of side effects was similar with each drug. Nalbuphine is a useful postoperative analgesic, as effective as pethidine. Nalbuphine 15 mg is apparently equipotent with morphine 10 mg or pethidine 120 mg by this mode of administration.