Methadone, a potent opioid analgesic, has long been considered a μ-opioid, based upon the similarities between its actions and those of morphine. This classification is supported by the sensitivity of methadone analgesia to the highly μ-opioid receptor-selective antagonist β-funaltrexamine. Yet, CXBK mice respond normally to methadone despite their insensitivity to systemic morphine, distinguishing between the receptor mechanisms of the two drugs. β-Funaltrexamine antagonizes methadone analgesia in CXBK mice, implying that the opioid is still acting through a μ-opioid receptor. These results reveal distinct analgesic mechanisms for morphine and methadone and provide further support for multiple subtypes of μ-opioid receptors.