dl-N-(2-Hydroxy-3-napthyloxypropyl)-N′-dansylethylenediamine, dansyl analogue of propranolol (DAPN) is a novel fluorescent β-adrenergic antagonist with high affinity to β-receptors. The distribution pattern of DAPN fluorescence was studied in the rat central nervous system subsequent to its intravenous administration to living rats. DAPN distinctly labels specific regions and cells in the central nervous system (CNS). Highly dense DAPN fluorescence was observed in the pyramidal cell layer of the hippocampus, the granule cell layerof the dentate gyrus, the basal layers of the piriform cortex and the neocortex, the cerebellar Purkinje cell layer, and the spinal α-motoneurons. Pretreatment of control rats with dl- and l-propranolol markedly decreased the intensity and density of DAPN fluoresence in the tissue sections, whereas prior administration of d-propranolol had almost no effect. Pretreatment with large doses of reserpine did not alter the pattern of DAPN fluorescence. These findings were identical to those observed with another fluorescent β-blocker, 9-aminoacridino-propranolol (9-AAP). Our data suggest that fluorescent β-adrenergic antagonists may be used in vivo for the direct probing of the β-receptors within the mammalian CNS.