The "mild" periodic acid-thionine (mPAT)-method was examined for the quantitative research of histochemical localization of sialic acid (NeuAc)-containing compounds in brain samples of goldfish and albino mice in comparison to salivary gland (Glandula submandibularis) of the mouse. Biochemical determinations of NeuAc contents of the tissues showed that during histological treatment a significant decrease in the amount of lipid-bound NeuAc, especially in the brain had occurred, because of its high content of lipid-bound NeuAc. A previous KOH-treatment caused in an increase of the colour reaction, which indicated mainly the amount of O-acylated NeuAc just as possibly lactone containing NeuAc-compounds. The intensity of staining, especially in the optic tectum of goldfish, decreased to 45% after neuraminidase-treatment. The low concentration of NeuAc-compounds in the brain in comparison to the salivary glands, results in only a faint staining. Therefore, the use of the mPAT-method seems to be not very suitable for a specific and quantitative staining of all NeuAc-compounds in the CNS.