Abstract: AimTo compare the antagonistic effects of 6β-naltrexol and naltrexone against morphine analgesia. MethodsThe effects of 6β-naltrexol and naltrexone against morphine analgesia were observed in mouse heat radiant tail-flick assay and in mouse (55±1) ℃ hot plate test. The displacement of 6β-naltrexol and naltrexone on binding to CHO-mu receptor was observed by radioligand binding study. Results6β-naltrexol antagonized morphine analgesia but the potency was (6.1±1.7)% that of naltrexone. The effective duration of 6β-naltrexol was 3-4 times that of naltrexone and the peak time of the response was about 0.5-1 h after sc equivalent efficacy dose (ED₉₅) in two models. Like naltrexone, 6β-naltrexol was effective by oral administration and the potency ratio of po/sc was 1/3. As an antagonist to opioid receptor, the displacement of 6β-naltrexol was about 12.5% that of naltrexone, which was almost in agreement with the efficacies against morphine analgesia in mouse. ConclusionCompared with naltrexone, 6β-naltrexol was less potent but duration was longer.