Tegafur and UFT are most widely used for the treatment of cancers of the gastrointestinal tract. FdUMP, a metabolite of 5-fluorouracil(5-FU), is known to have an inhibitory activity of thymidylate synthase (TS). We administered 600 mg/day of tegafur or UFT to 37 patients with cancer of the large bowel beginning 1 week prior to surgery and then measured the concentrations of tegafur and 5-FU, and TS inhibition rate in the excised tissue. Our results showed that the concentrations of 5-FU in tumor were 0.077 +/- 0.026 microgram/g in tegafur group and 0.208 +/- 0.143 microgram/g in UFT group, with the UFT showing significantly higher levels (p less than 0.05). On the other hand, TS inhibition rates in tumor tissue were 45.5 +/- 13.3% in tegafur group and 56.1 +/- 13.0% in UFT group, with a significantly higher level existing in the latter group (p less than 0.05). Furthermore, the same high 5-FU concentration and TS inhibition rates were observed in the lymph nodes affected by metastasis. No difference between the tegafur and UFT groups was noted in normal tissue or normal lymph nodes, and compared to tegafur, UFT showed an effective action on cancerous large bowel tissue.