In addition to a mucolytic action, ambroxol has antioxidant and anti-inflammatory properties. The antioxidant effects of ambroxol were studied both in vitro and in vivo. In vitro methods, such as (1) inhibition of hyaluronic acid degradation induced by hydroxy radicals and (2) standard lipid peroxidation assay in rat liver mitochondria and gastric mucosa, induced by tert-butyl hydroperoxide, were used. The in vivo approach was based on the study of the protective effect of pretreatment with ambroxol in a rat model of gastric corpus and antral lesions, induced by indomethacin. The inhibition of the degradation of hyaluronic acid was measured as a change of its viscosity; ambroxol (1,000 microl/l) reduced the degradation by 93%. Lipid peroxidation with tert-butyl hydroperoxide as a source of radicals was followed by the formation of thiobarbituric acid reactive substances. Ambroxol (10 mmol/l) inhibited lipid peroxidation by 96% in the rat liver mitochondria, and by 74% in the gastric mucosa. In vivo, ambroxol was administered p.o. at a dose of 10, 30, and 50 mg/kg, at 5, 30, and 60 min prior to indomethacin administration. The highest inhibition of the number of corpus gastric lesions and lowering of the lesion index (38% and 62%, respectively) was shown after the administration of 50 mg/kg, 30 min before indomethacin administration. Antral lesions were inhibited to a lesser extent by the same dose of ambroxol, administered 30 min before indomethacin treatment. Inhibition of the number of antral lesions reached 27% and the total area of the gastric damage was even larger (the ulcer index reached -5%).