It has been found in clinical practice that the serum level of phenytoin, of which metabolism is mediated by hepatic CYP2C enzymes, was markedly elevated by co-administration of 5-fluorouracil (5-FU) and doxifluridine (5'-deoxy-5-fluorouridine; 5'-DFUR), a prodrug of 5-FU, but the detailed mechanisms are unclear. A study using rats was undertaken to examine the effects of 5-FU and 5'-DFUR on phenytoin metabolism in hepatic microsomes and phenytoin pharmacokinetics in-vivo. Neither 5-FU nor 5'-DFUR exhibited direct inhibitory effects on hepatic microsomal phenytoin p-hydroxylation, a major metabolic route catalysed by CYP2C in rats, as in humans. 5-FU and 5'-DFUR were injected intraperitoneally into male rats as single doses (1.68 mmol kg(-1)) and repeated doses (0.24 mmol kg(-1) for 7 days). Control rats received vehicle alone. A significant reduction in the activity of phenytoin p-hydroxylation was observed 4 days after the last administration irrespective of the agents and their treatment regimens, although the activity was unchanged on Day 1. Pharmacokinetic analysis of phenytoin revealed that the elimination rate constant and the total clearance was decreased by 70-75% in both the 5'-DFUR-treated and 5-FU-treated rats, indicating that the decrease in the metabolic capacity of phenytoin was responsible for the change in phenytoin disposition in-vivo. On the other hand, 5-FU significantly depressed the total P450 content, NADPH cytochrome c reductase activity and activities of progesterone hydroxylations. However, the depressive effects of 5'-DFUR were not very potent relative to those of 5-FU, which can be explained by the fact that 5-FU is derived from 5'-DFUR to only a small extent. According to a recent report, phenytoin p-hydroxylation and progesterone 2alpha-/21-hydroxylations share common CYP2C enzymes as their catalysts. Because there was a difference in the modulation profiles between phenytoin p-hydroxylation and progesterone 2alpha-/21-hydroxylations after exposure to 5'-DFUR, 5'-DFUR might modulate phenytoin metabolism without loss of catalytic ability for other substrates, unlike 5-FU. The present study suggested that the down-regulation of hepatic CYP2C enzymes occurs by 5-FU exposure even at a low level, and provided a fundamental explanation for the drug interaction encountered in clinical practice.