Pneumoconioses produced by intratracheal applications of various dusts (quartz, coal, cadmium and lead sulfide) in rats were investigated by electron microscopy in order to follow the pathway of the dust particles from the alveoli into the pulmonary interstitium.As postulated by Spencer in 1977 on the basis of light microscopic investigations, the dust particles produce necroses of the alveolar septae (‘alveolar ulcers’). Two forms of necroses appear to occur: with a less severe dust exposure, individual pneumocytes and their basement membrane are destroyed by dust particles. Dust-laden macrophages are deposited here which are displaced into the stroma after re-epithelization of the alveolar defect. On the other hand, with massive dust exposure, almost all pneumocytes of the affected alveoli become necrotic. The affected alveoli collapse and are replaced by connective tissue, so that the dust is situated in the connective tissue stroma.A transcellular penetration of the dust particles into the pulmonary interstitium or an immigration of dust-laden macrophages into the pulmonary stroma through the intercellular junctions of intact pneumocytes was not observed in any of the pneumoconiosis mod