TAS-102, a new oral drug, is composed of an antitumor drug, alpha,alpha,alpha-trifluorothymidine (FTD), and its metabolic inhibitor, 5-chloro-6-(2-iminopyrrolidine-1-yl)methyl-2,4(1H,3H)-pyrimidinedione hydrochloride (TPI). It has been reported that the oral administration of TAS-102 increases the AUC of FTD in rodents and monkeys in different manners. In this study, a pharmacokinetic model was developed, in an attempt to evaluate the bioavailability of FTD in these animals after the co-administration of TPI. Since TPI inhibits FTD metabolism competitively, a time-dependent as well as concentration-dependent model for the hepatic intrinsic clearance of FTD was developed including the time courses of both FTD and TPI. Based on this modeling, we were able to quantitatively explain the TPI dose-dependent enhancement of AUC of FTD in monkeys, while little increase was observed in rats. These results are consistent to observations that thymidine phosphorylase (TPase) is predominantly expressed in monkeys; while uridine phosphorylase (UPase) is superior to TPase in rats. Since TPase is also predominantly expressed in humans, the pharmacokinetic model developed in this study can be used to explain the bioavailability of TAS-102 in humans.