Pharmacokinetic behaviors of 5-FU and masked fluoropyrimidines were reviewed. Anabolic and catabolic pathways of 5-FU Pharmacokinetics of 5-FU-blood level, absorption, drug interaction Tegafur (FT)-blood level, absorption, activation to 5-FU by P-450 UFT-high 5-FU level in tissues, especially in tumor tissues, by inhibiting the catabolism of 5-FU from FT with uracil HCFU-relatively high 5-FU production, independent of the liver, high drug distribution in cancerous ascites 5'-DFUR-activation to 5-FU by uridine- or thymidine- phosphorylase, high 5-FU level in tumor tissues