Maximum Tolerated Dose of Tegafur was estimated by per-oral administration in male Donryu rats. Tegafur was administered for 24 weeks at respective doses of 90 mg, 120 mg and 150 mg/kg/day. Six of 10 (60%) died during the treatment in those groups administered with 120 mg and 150 mg/kg/day, respectively. The cause of death of these rats was severe pneumonia. All 10 of the group given 60 mg/kg/day survived until end of the administration and were sacrificed for histological examination of organs. These rats showed apparent suppression of body weight gain compared with controls (p less than 0.001). Slight inflammation of the lungs was observed in all rats, congestion of the liver and local degeneration and necrosis of the liver cells in a few rats and cellular degeneration of bone marrow in one rat. There was no remarkable change in the digestive tract, kidney, thymus or reproductive organs. Accordingly, the Maximum Tolerated Dose of Tegafur was determined to be 90 mg/kg/day.