A new fluorescent beta-adrenergic antagonist, 9-amino-acridin propranolol (9-AAP), was administered intravenously to living mice. In the cerebral cortex, the highest concentration of 9-AAP was noted in the hippocampal formation, where it distinctly labeled the hippocampal pyramidal cell layer and the granule cell layer of the dentate gryus. High uptake occurred in the pyramidal cell layer of the piriform cortex. In the neocortex, fluorescence was less dense and more diffuse but confined to the basal layers. A similar pattern was observed in the basal layers of the cingulate cortex, but an additional high-density dotted fluorescence labeled its layer II. In the cerebellar cortex, 9-AAP was localized within the Purkinje cell layer. In the spinal cord, the highest density of fluorescence was observed in the nuclear collections of alpha-motoneurons. The findings were similar to those observed in the central nervous system of the rat and support the reproducibility of the method. 9-AAP may be used in vivo as a fluorescent probe to map out the central beta-adrenergic receptor system.