Rats were randomly placed in one of two groups. The first group was exercise conditioned on a treadmill for four weeks; the second group remained sedentary over the same time period. The exercise conditioned rats were exposed to either DB 1/88 cotton dust (6.1 ±0.5 mg/m³) or filtered air as they were periodically exercised on a treadmill. Similarly, nonexercising rats were exposed to DB/188 cotton dust (4.9 ±0.4 mg/m³) or filtered air. Cells from the lungs of each animal were harvested 6 hrs post exposure by bronchoalveolar lavage. The total number of cells per animal was determined as well as the number of granulocytes, lymphocytes, erythrocytes and macrophages using differential cell counting techniques. In addition, the superoxide produced by macrophages in response to zymosan stimulation was determined. It was found that the total number of cells and the number of granulocytes, lymphocytes and erythrocytes were significantly elevated in exercising rats compared to sedentary animals when both groups were exposed to similar concentrations of cotton dust. There was also a significant increase in the production of superoxide in macrophages of exercising animals exposed to cotton dust. The number of macrophages was not significantly different between exercised and sedentary animals exposed to cotton dust or filtered air. This study shows that exercise during exposure significantly affects the pulmonary cellular response of rats to a given mass concentration of cotton dust. It is likely that the response to the dust is potentiated by an increase in the deposition of the dust in the airways and lung parenchyma as a result of increased ventilation rate and alterations in the regional distribution of the dust due to exercise.