Meptazinol is an agonist-antagonist opioid analgesic believed to be unique in its selectivity for mu1 (high affinity) receptors and its cholinergic activity. Our objectives were to determine the relative analgesic potency of intramuscular meptazinol and morphine and to compare mood and side effects in 102 patients with cancer who have postoperative pain. Meptazinol (50, 100, and 200 mg) and morphine (4, 8, and 16 mg) were given for moderate to severe pain in a double-blind, randomized but balanced, incomplete block design. Serial multiple assessments of pain, relief, mood, and side effects were made. The most precise estimates of relative analgesic potency indicate that meptazinol is equivalent to 10 mg morphine at 120 mg (95% confidence interval 80 to 170 mg) for peak effect and at 175 mg (95% confidence interval 125 to 270 mg) for total effect. Mean (+/- SE) times to peak effect and to remedication were 0.9 +/- 0.1 and 3.6 +/- 0.2 hours for meptazinol and 1.4 +/- 0.1 and 4.8 +/- 0.4 hours for morphine at equianalgesic peak effects. The percentages of subjects with one or more side effects were 18, 49, and 73 for graded meptazinol doses and 32, 49, and 65 for graded morphine doses. Mean numbers of side effects per subject were 0.3, 1.5, and 3.5 for meptazinol and 0.5, 0.7, and 1.7 for morphine. Profiles of side effects differed. Mood improvement and overall satisfaction were dose related and greater for morphine than for meptazinol. Side effects may limit the use of meptazinol in doses that relieve severe postoperative pain.