Two kinds of sepiolite, a $500^{circ}C$ heat-treated sepiolite, and a $700^{circ}C$ heat-treated sepiolite were analyzed for their physicochemical properties. After these sepiolites were instilled into rat lungs, the effects of the substances on lung pathological changes were evaluated. The lungs instilled with sepiolite increased their weight compared with the unexposed control. The pathological examination further showed increased legions of granuloma with early fibrosis. The heat treated sepiolites, however, did not show any toxicological differences from the untreated sepiolites. Thus chronic experiments are needed to evaluate the durability of mineral fibers, which is an essential experiment for evaluating biopersistence of fibers in lungs.