The rapid metabolism of heroin to 6-acetylmorphine and its slower conversion to morphine has led many to believe that heroin and morphine act through the same receptors and that the differences between them are due to their pharmacokinetics. We now present evidence strongly implying that heroin and two potent mu drugs, fentanyl and etonitazine, act through a unique receptor mechanism similar to morphine-6 beta-glucuronide which is readily distinguished from morphine. Heroin, 6-acetylmorphine and morphine-6 beta-glucuronide show no analgesic cross tolerance to morphine in a daily administration paradigm, implying distinct receptors. Strains also reveal analgesic differences among the drugs. CXBK mice, which are insensitive to morphine, retain their analgesic sensitivity to heroin, 6-acetylmorphine, morphine-6 beta-glucuronide, fentanyl and etonitazine. Antisense mapping of the mu opioid receptor MOR-1 reveals that oligodeoxynucleotide probes against exon 2, which are inactive against morphine analgesia, block morphine-6 beta-glucuronide, heroin, fentanyl and etonitazine analgesia. Finally, an antisense probe targeting Gi alpha 1 blocks both heroin and morphine-6 beta-glucuronide, but not morphine, analgesia. These results indicate that heroin, 6-acetylmorphine, fentanyl and etonitazine all can produce analgesia through a novel mu analgesic system which is similar to that activated by morphine-6 beta-glucuronide.