The effect of combined tegafur and uracil (UFT) on the development of rat urinary bladder tumors induced by N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) was studied. Two hundred F344 male rats were divided into 10 groups. Groups 1 to 5 were given 0.05% BBN in drinking water for the initial 8 weeks of the experiment, and Groups 6 to 10 were the controls of the prior 5 groups treated with BBN. UFT in commercial diet was administered at daily doses of 100mg (30.9 mg as FT-207) per kg of body weight in Groups 2 and 4 and 200 mg (61.7 mg as FT-207) per kg of body weight in Groups 3 and 5. Groups 2 and 3 received UFT throughout the period of the experiment, and Groups 4 and 5 for 12 weeks after 8 week treatment with BBN. All animals were sacrificed at 20 weeks, and studied histopathologically. In Groups 1 to 5, urinary bladder tumors developed in 20 of 20, 13 of 20, 6 of 20, 14 of 20 and 6 of 20, respectively. Incidences of tumors in the 4 groups treated with UFT were significantly lower than that in Group 1 treated with BBN alone. This result shows that UFT inhibits the development of urinary bladder tumors in rats induced by BBN.