Rats inhaled by nose-only exposure an aerosol of fused aluminosilicate particles, 3.95 mu m activity median aerodynamic diameter, labelled with Co-57. The amount remaining on the airway surface at 7,28 and 112 days after inhalation was determined at sacrifice by measuring the Co-57 content in successive washes of trachea and of the region of the first bifurcation. Similar low levels were found in both cases. From Co-57 excretion measurements, the rate at which particles were being cleared through the trachea was calculated, and hence the amount on the tracheal surface that was expected from rapid mucociliary clearance, T-f. As expected, T-f decreased markedly during the study period. However, at all times the amount of Co-57 washed from the tracheal surface was greater than T-f. The difference was a measure of the amount remaining on the surface long-term, T-s. At 7 and 28 days after inhalation, T-s was approx. 4 x 10(-4) of the thoracic content at 1 day, decreasing to 7 x 10(-5) after 112 days. The fraction sequestered in the tracheal wall, T-w, remained approximately constant at around 1 x 10(-4). At 7 days, T-s and T-w were 1.81 and 0.45%, respectively, of the estimated amount deposited on the trachea. Much of the Co-57 activity in the tracheal washes was associated with macrophages, the proportion increasing with time.