The described method for selective staining of neurons in the central nervous system of vertebrates and invertebrates involves the application of minute crystals of methylene blue or small amounts of dye solution to the tissue. Staining, however, depends on the diffusion qualities of the dye and not on active axonal transport mechanisms. Furthermore, the production of air-dried frozen sections after fixation by ammonium heptamolybdate is considered the best procedure for exact assessment of the results. This method is well-suited for routine use in both neuroanatomic and electrophysiologic research.