1,3-Bis(tetrahydro-2-furanyl)-5-fluorouracil (FD-1) is a new masked compound of fluorinated pyrimidine and a derivative of 1-(tetrahydro-2-furanyl)-5-fluorouracil (FT-207). The pharmacokinetics of FD-1 and FT-207 were compared in the livers and kidneys of control mice and in mice (DD males) pretreated with phenobarbital. The half-time in the liver of FD-1 orally administered was about 40 minutes, whereas that of FT-207 in the liver was about 3 hours. Hepatic concentrations of 5-fluorouracil (FUra) originating from FD-1 were three to five times as much as those originating from FT-207. FD-1 formed 3-(tetrahydro-2-furanyl)-5-fluorouracil (3-T-F-FU) and FT-207 by a ratio of about 3 to 10, respectively. Although FD-1 had a short period of half reduction, FT-207 and 3-T-F-FU had a half-time lasting for 3--4 hours. In contrast, the renal concentration of FD-1 was one-third that of FT-207. Oral administration of FD-1 to the mice pretreated with phenobarbital elevated the FT-207 and 3-T-F-FU levels in the livers to twice the levels in the control mice and further elevated the FUra levels in the livers to twice the levels in the controls. These results indicate that FD-1 is catabolized in the liver by microsomal enzymes (including cytochrome P450) faster than is FT-207, which consequently enlarges the hepatic pool of the intermediates on the way to FUra formation.