The analgesic profile and side-effects of buprenorphine 0.3 mg and morphine 10 mg intramuscularly were compared postoperatively in a double-blind, non-crossover, multiple-dose study. When the patient complained of moderate to severe postoperative pain after halothane-relaxant anesthesia for upper abdominal surgery, the first test dose of either drug was given. Subsequent similar doses of buprenorphine 0.3 mg or morphine 10 mg were given when required (maximum ten doses). The first dose of both drugs gave an equal decrease in pain intensity, suggesting a relative potency of 33:1 for both buprenorphine/morphine. A mean of 0.51 mg buprenorphine or 17 mg morphine had to be administered for satisfactory initial analgesia. Thereafter, the next analgesic dose was required a mean of 10.3 h after buprenorphine compared to 5.9 h after morphine (P less than 0.01). Significantly (P less than 0.01) fewer analgesic doses (mean 5.6) were needed in the buprenorphine group within the first 48 h postoperatively as compared to the morphine group (mean 7.3). A more pronounced mean decrease in the respiratory rate was observed after buprenorphine, but the mean minimum respiratory rates did not differ significantly from each other. Other effects of the two drugs on vital signs were similar. The incidence of other side-effects was fairly similar after both analgesics. The patients' subjective appraisal favoured buprenorphine.