Fly ash (30 mg) from an experimental fluidized bed coal combustor (FBC) was intratracheally instilled once into male Syrian golden hamsters. At 1, 3, 6, 9, or 30 days postadministration, lungs were fixed by intratracheal perfusion and tissues were processed for light and electron microscopic evaluation. Standard point count morphometry, used to analyze distal lung structures, revealed: an increase in volume density (Vv) of noncellular interstitial space at Day 1 which remained elevated at Day 3 but returned to control values subsequently; increased Vv for polymorphonuclear leukocytes (PMNs) and alveolar macrophages in alveolar air space at Day 6; increased Vv of cellular interstitium at Day 9; and increased Vv of noncellular interstitium at Day 30 following exposure. These observations are consistent with the hypothesis that the primary response of the distal lung to particle exposure is recruitment of PMNs and macrophages. When activated, these cells may release substances affecting the cellular and noncellular composition of the interstitial space, leading eventually to increased connective tissue in the interstitium.