1.Naloxazone, a hydrazone derivative of naloxone which blocks mouse brain opiate receptorsin vivofor over 24 hr, produced a prolonged blockade of opiate receptor binding to rat brain membranesin vitro.At low concentrations (1-100 nM), naloxazone reversibly displaced [3H]naloxone binding. Incubation of brain homogenatesin vitrowith 1-10µMnaloxazone resulted in a loss of 40–60% of specific [3H]naloxone binding after four cycles of extensive washing or after extensive dialysis which removed 100% of similar concentrations of naloxone.2.Treatment with 5µMnaloxazone also blocked binding of several3H-labeled opiate agonists and agonist-antagonists with no effect on muscarinic cholinergic orβ-adrenergic binding. Scatchard analysis of3H-labeled opiate binding after naloxazone pretreatment revealed a selective loss of high-affinity binding sites with minimal effects on low-affinity binding.3.Low-affinity sites remaining after naloxazone treatment were regulated by GTP and sodium like high-affinity s