Flumazenil is an imidazobenzodiazepine, an antagonist of central benzodiazepine (BDZ) receptors. BDZ binding sites are a modulatory component located on the gamma-aminobutyric acid (GABA) receptor macromolecule. We studied the effect of monocular enucleation on [3Hlflumazenil binding in deprived and intact visual areas and nonvisual areas of the adult mouse brain under in vivo conditions. [3Hlflumazenil binding was examined at seven time points up to 56 days postenucleation. In some monocularly deprived mice, changes in local blood flow accompanied with the BDZ receptor response were evaluated by coinjection of [3Hlflumazenil and 99mTc-HMPA0. Monocular enucleation produced a transient increase in [3H]flumazenil binding in the deprived visual cortex and superior colliculus. At 17 days postenucleation, [3Hlflumazenil binding in the anterior and posterior portions of the visual cortex and the superior colliculus increased by 28%, 15% and 23%, respectively, and declined to control levels at 45 days postenucleation. The increase in [3Hlflumazenil was accompanied with a decrease in blood flow. Alterations in BDZ receptors and blood flow were selective to deprived visual structures. The regional correlation between the metabolic deficit and the BDZ response provides further support that the increase in BDZ receptor binding is confined to regions of reduced neuronal activity. [llClflumazenil is an excellent radiotracer for in vivo imaging of benzodiazepine receptors in human brain using positron emission tomography (PET). This study suggests the suitability of [llC]flumazenil for in vivo PET study of BDZ receptor response to deafferentation of visual structures in human brain.