This paper presents a technique for glyoxylic acid-induced monoamine histofluorescence in the central nervous system. Unperfused rat brains are sectioned in a cryostat, immersed in 2% glyoxylic acid solution, warm-air dried and gassed at 100 degrees C. Intense, well-localized catecholamine fluorescence is produced and all known catecholamine-containing structures are demonstrated. The fluorescence obtained by this method was evaluated by a variety of agents and was shown to be catecholaminergic in origin. In contrast to the Vibratome-glyoxylic acid technique, this procedure reliably produces thin, whole-brain sections of even thickness and allows protracted use of the tissue block. Because unperfused tissue is used, the technique can be applied to a broad variety of material, such as post-mortem tissue or invertebrate preparations. Alternate sections can be prepared for a variety of techniques requiring unperfused tissue (e.g., enzymatic localization, chemical assay, anatomical techniques). The reasons for choosing each of the parameters in the technique are discussed.