THE presence of benzodiazepine receptors has been demonstrated using isolated membrane preparations from the mammalian central nervous system 1-3. Specific binding of 3H-diazepam to synaptosomal membranes is maximal at temperatures of 0-4 °C, but reduced by more than 95% at physiological temperatures4. Although the inhibition of 3H-diazepam binding by other benzodiazepines in vitro closely parallels their pharmacological potencies in vivo, failure to demonstrate specific binding in physiological conditions raises questions as to whether receptor binding occurs in vivo. We report here that benzodiazepine receptors can be demonstrated in the intact animal following intravenous administration of 3H-diazepam. The labelling of benzodiazepine receptors in vivo permits studies of receptor occupancy in various pharmacological and physiological conditions as well as autoradiographic localisation of the receptor in situ.