The aim of this study was to investigate the effect of heparin on the rat bronchial mucosa changes induced by sulphur dioxide (SO2) inhalation. Sixty five rats were used in this experiment. Five of them constituted a control group, while 60 were exposed to SO2. Forty of the latter subgroup were additionally treated with low molecular weight heparin (LMWH), either during or after terminating exposure to SO2. In all animals exposed to SO2 inflammatory cells were found in broncho-alveolar lavage fluid (BALf) in numbers significantly higher from those observed in healthy controls. The rats exposed to SO2 and treated with LMWH showed intermediate cell pattern in the bronchi between healthy and SO2- exposed animals. When comparing histological picture of the bronchi, we noted extensive changes in irritated rats. These changes were either less expressed or totally absent in animals treated with heparin. The activity of enzymes: acid phosphatase (ACP), alkaline phosphatase (AP) and lactate dehydrogenase (LDH) rose in BALf, although the rise was not parallel and did not correlate with the magnitude of cellular influx or histological changes. Heparin did not influence this changes.