Naloxazone, a hydrazone derivative of the opiate antagonist naloxone, has a high affinity for opiate receptor binding sites. Naloxazone injections reduce opiate receptor binding to extensively washed mouse brain membranes for more than 24 hours, suggesting that the effect is irreversible. High-affinity binding sites are abolished by this treatment, whereas low-affinity sites are unaffected. Naloxazone treatment blocks the analgesic effects of morphine for at least 24 hours but does not prevent death from high doses of morphine. Thus analgesic but nonlethal opiate effects may be mediated by the high-affinity subpopulation of opiate receptors.