One hundred-fifty post-operative adult patients with moderate to severe pain were enrolled into this analgesic efficacy study comparing single doses of tonazocine mesylate, a new mixed agonist-antagonist opioid analgesic, with morphine. The patients were randomly assigned to five treatment groups: tonazocine mesylate 2, 4, 8 mg; morphine sulfate 10 mg and a placebo group. The results showed mean total pain relief scores for tonazocine 4 mg were nearly identical with that of morphine sulfate 10 mg while 8 mg of tonazocine were superior to 10 mg of morphine. All the active medication groups were superior to the placebo group (P less than 0.02) for both pain intensity and pain relief. Relative potency determined by the dose response indicates that 3.2 mg of tonazocine is equivalent to 10 mg of morphine. Drowsiness was the main adverse reaction seen in all active treatment groups. Tonazocine mesylate appears to be a potent analgesic with promising clinical usefulness and warrants further study.