Fischer 344 rats were exposed for 1 hr to an aerosol of BeO generated at a temperature of 560 degrees C. An initial lung burden of 500 +/- 4.1 ng Be was achieved. Animals were killed at 2.5 hr, and 2, 12, and 21 days postexposure. Bronchopulmonary lavage fluids were analyzed biochemically for enzymes, protein, lipids, and sialic acid, and cytologically to determine the composition of the free alveolar cell population. Nonspecific phagocytosis of yeast was measured in adherent macrophages. There were increases in all the biochemical parameters by 2 days postexposure, which peaked by Day 5 and then began to return to control levels. The cytological response on Days 2 and 5 was characterized by polymorphonuclear leucocyte infiltration and a depression in macrophage number and phagocytic activity. By Day 12, increased numbers of newly recruited macrophages with supranormal phagocytic activity populated the lung. During the same period, there was a reduction in lavage protein and lipid levels, perhaps due to a restoration of normal clearance mechanisms. Tissue morphological changes correlated well with the cytological and biochemical alterations.