Laboratory rats were exposed to the inhalation of dust from an agglomeration unit which is the greatest contributor to dust pollution in the vicinity of a mercury producing plant. The exposure lasted for 6 months (4 hours daily, 5 days per week), the concentration of aerosol in the chamber was 10 mg.m-3. After finishing the exposure, the animals were examined and compared with the controls which were held under standard laboratory conditions. The number of alveolar macrophages was highly elevated (P<0.001) in the exposed animals, Mg24 ATPase activity in the heart muscle was decreased. The alanine aminotransferase activity in the serum was not changed, the aspartate aminotransferase was slightly enhanced. No differences in the frequency of abnormal sperm and in the frequency of polychromatic erythrocytes in bone marrow were detected.