Titan dust containing about 50% Ti, and titanium hydride (TiH2) con-taining 99.97% Ti were inhaled by rats and rabbits. TiH2 dust was infused intra-tracheally in rats, and the effects of titan dust on the laboratory animals were tested. Rats and rabbits were exposed to titan dust at concentrations of 22.4, 70.5, and 476 mg/m3 for periods ranging from 1 month to about one year, and those surviving were observed for about one year. In the case of TiH2 dust, rats and rabbits were exposed to a concentration of around 529 mg/m3 for priods of 2 to 8 months, the TiH2 dust being infused intratracheally in the case of rats. Radio-logically in rabbits, obvious nodular shadows were revealed in the chests of rabbits exposed to titan dust, and such findings increased chronologically. Histopatho-logically, retention of the fine dust in alveoli and lymph nodes, proliferation of the alveolar walls, hyperplasia of connective tissue cells, and phagocytes and macrophages containing absorbed dust particles were found together with fibrosis. Suspensions of the TiH2 dust in physiological saline solution containing 10, 50, 100, and 500 mg in 1 ml respectively, were infused intratracheally in 39 male rats. In this intratracheal infusion group and the TiH2 inhalation group changes in bron-chioli, lung and lymph nodes, dust cells and giant cells, bronchopneumonia and granulomatous pneumonia, and intraalveolar fibrosis were shown. As a result of these investigation, it was concluded that the inhalation of titan dust or titan compounds such as TiH2 can produce respiratory disorders and pulmonary fibrosis (titanosis) in laboratory animals.