The relationship between the antitumor activity and the inhibition of thymidylate synthase after oral administration of 5-FU, FT-207 or UFT was examined. The inhibition of tumor growth on sarcoma-180 after oral administration of 5-FU (20 mg/kg), FT-207 (120 mg/kg) or UFT (30 mg/kg) was 40%, 50% or 70%, respectively. It was found that the inhibition of thymidylate synthase in sarcoma-180 tumor tissue after single oral administration of 5-FU (20 mg/kg), FT-207 (30 or 120 mg/kg) or UFT (30 mg/kg) were about 45%, 20%, 50% or 65%, respectively, but the activities of other enzymes involved in DNA synthesis were not almost inhibited. Thymidylate synthase was inhibited more severely by oral administration of UFT (30 mg/kg). The activities of other enzymes were not inhibited even by continuous oral administration of these drugs for 6 or 11 days. The extent of inhibition of thymidylate synthase seemed to be parallel to that of inhibition of tumor growth. These results suggest that the inhibition of thymidylate synthase by FdUMP converted from 5-FU, FT-207 or UFT plays a major role in their antitumor actions.