Difficulties encountered in the analysis of the monofluorinated amphetamine, 2-amino-3-fluoro-1-phenylpropane (AFP), are discussed. 2. A novel assay procedure and its application to the analysis of AFP in the rat brain are described. The procedure involves pentafluorobenzoylation of AFP under aqueous conditions and quantitation by a gas chromatography equipped with a capillary column and an electron-capture detector.