The 14-hydroxydihydromorphinone hydrazones have been quite useful in studying aspects of opiate receptor binding and function. The most extensively studied, naloxazone, effectively and selectively inhibits high affinity (mu1) sites in vitro and morphine analgesia in vivo. We now report on the actions of oxymorphazone on receptor binding in vitro and on analgesia in vivo. Oxymorphazone effectively lowers 3H-opioid binding despite extensive washes with the same selectively for high affinity, or mu1, binding sites as naloxazone. Acutely, oxymorphazone was less potent in vivo than oxymorphone (ED50's of 0.8 and 0.4 mg/kg, respectively). Both quantal dose response curves were parallel. At higher doses (100 mg/kg), 83% of animals given oxymorphazone over 20 hr previously were analgesic whereas none of the oxymorphone animals were (p less than 0.001). Oxymorphazone also produced prolonged analgesia after icv administration. These results, in addition to other studies, suggest that oxymorphazone's actions cannot be adequately explained by pharmacokinetic differences from oxymorphone and supports the hypothesis that the drug works via prolonged receptor binding.