Fibrogenic effect of two natural amorphous silica dusts-diatomite from deposits near Leszczawka (Poland) and silica earth (from the USA) was tested on the rats. Pneumoconiosis was produced by intratracheal introduction of 50 mg dust, at a single dose. Fibrogenic effect was assessed after 3, 6 and 9 months after introduction of dust. Analysis of diatomite dust carried out with X-ray diffraction method showed the presence of quartz in a quantity not exceeding 5%, while in silica earth dust from the USA no silica critalline structures were found. Development of fibrogenic changes in lungs of the rats assessed with hydroxyproline (collagen) increase and weight increment of wet lungs was small. The increment of the indices was twice higher as compared with their increment found in the lungs of control animals but lower than in the rats which were given cristalline silica dusts. In histopathological examinations no progressive lung fibrosis was found though some signs of destruction and necrobiosis in some cells were noticed.