The properties of a new fluorescence histochemical method for arylethylamines based on reaction with a mixture of 4% formaldehyde and 0.5% glutaraldehyde in aqueous solution are described. At room temperature the aldehyde mixture produced a well-localized fluorescence reaction in tissues, which, when examined microscopically in aqueous solution, was sufficiently intense for fine terminal noradrenergic axons to be seen. If the tissue was subsequently dried, the fluorescence intensity increased. At the same time as inducing the fluorophores, the aldehyde mixture fixed the tissue to a standard well suited for electron microscopy. It thus proved possible to locate amine containing cells in the fluorescence microscope and subsequently examine their ultrastructure. In aqueous models, the aldehyde mixture formed fluorescent products with adrenaline, noradrenaline, dopamine, dopa, 5-hydroxytryptamine and 5-hydroxytryptophan, but not with histamine or octopamine. The fluorescence induced in the aldehyde mixture remained stable if the tissue was subsequently transferred to saline or distilled water and when it was dehydrated in ethanol and cleared with xylene, benzene, chloroform or acetone.