1-Ethoxymethyl-5-fluorouracil (EM-FU) is a fluorinated pyrimidine derived from 5-FU, and 3-cyano-2,6-dihydroxypyridine (CNDP) is a chemical modulator which suppresses the catabolism of 5-FU by inhibiting dihydrouracil dehydrogenase in the liver. In this study, the metabolism of EM-FU and the suppression of 5-FU catabolism by CNDP were observed by in vivo 19F magnetic resonance spectroscopy in comparison with other similar drugs, because it is considered that the most effective mode of therapy using 5-FU is to suppress the catabolism of 5-FU in the liver and so to maintain for longer an effective blood level of 5-FU. The metabolism of EM-FU was very slow and the production of fluoro-beta-alanine was very low as compared to the case of tegafur. The catabolic suppression by CNDP was much stronger than that of uracil. Therefore co-administration of EM-FU and CNDP should suppress catabolism and maintain an effective blood level of 5-FU for a long period of time.