Biochemical examinations in serum, homogenates and microsomal fraction of the liver, as well as biochemical and morphological examinations of liver tissue were performed in rats of Wistar strain exposed chronically for 6 months to ammonium fluoride vapours. A part of animals was protected with a mixture of sodium salts of quercetin synthetized in Inorganic Chemistry Department of the Technical University at Rzeszów, and added to the standard feed in a dose of 5 and 20 mg/kg body weight. It has been disclosed that NH4F causes disturbances in the activities of enzymes (cholinesterases, transaminases, alkaline and acid phosphatase), a rise of bilirubin concentration in serum, as well as disturbances in lipid metabolism, an increase in the content of total lipids, and disorders of lipid metabolism--increase in the content of total lipids, cholesterol and triglycerides, with phospholipids being decreased. The biochemical changes are accompanied by the hepatic tissue lesion. The use of quercetin alleviates, to a considerable degree, the biochemical and morphological disturbances due to protracted exposure to ammonium fluoride vapours.