A method for selectively enhancing the analgesic potency of a bimodally-acting opioid agonist selected from the group consisting of morphine, codeine, fentanyl, buprenorphine, methadone, enkephalin, dynorphin, and endorphin and simultaneously attenuating the undesirable excitatory side-effects caused by said bimodally-acting opioid agonist in a subject treated with said bimodally-acting opioid agonist comprising administering to the subject in combination with said bimodally-acting opioid agonist an amount of an excitatory opioid receptor antagonist selected from the group consisting of etorphine, dihydroetorphine and diprenorphine effective to enhance the analgesic potency of said bimodally-acting opioid agonist and attenuate the undesirable excitatory side-effects caused by said bimodally-acting opioid agonist.