Quinacrine (Atebrin), an antimalarial drug that binds to adenine nucleotides, was used to label paraneurons. The drug was dissolved in physiological saline and intraperitoneally injected (rats, mice, guinea pigs) in concentrations of 120 or 200 mg/kg body weight. The animals were killed three days later and paraneurons were studied in the fluorescence microscope aftre freeze drying. Bright fluorescence is observed in adrenomedullary cells, SIF cells, carotid body chief cells, pancreatic islet cells, anterior pituitary cells, pinealocytes, and mast cells. Weak fluorescence occurs in thyroid parafollicular Merkel cells, and a few entero-endocrine cells. It is suggested that quinacrine binds to ATP or related adenine nucleotides which are stored in secretory granules of paraneurons. Varying fluorescence intensity seems to depend on different concentrations of adenine nucleotides within the storage granules, as well as on the different size and number of these granules in various paraneurons. Besides formaldehyde-induced fluorescence of biogenic amines, HCl-basic dye staining methods and electron microscopy, the method is useful to identify and define paraneurons.