The pharmacokinetics of ftorafur (FT), an antineoplastic agent, has been studied in seven cancer patients by determining concentrations of the unchanged compound in serum after single IV and PO doses of 2 g FT. Serum drug concentrations were determined by a new quantitative thin-layer chromatographic method. After IV administration, the mean half-lives of the distribution phase and elimination phase were 1.0 h and 7.6 h, respectively. Total serum clearance was 69 ml/h . kg and the apparent volume of distribution was 0.66 l/kg. Following PO administration there was a short lag-time, 11 min, before the appearance of FT in peripheral serum, and the maximum concentration in peripheral serum was achieved in 3.2 h. Oral absorption was complete and no significant first-pass metabolism could be observed. FT elimination, measured in serum taken from the portal vein and a peripheral vein, occurred substantially at the same rate after IV and PO administration. In contrast, after the PO dose FT appeared in the portal serum significantly earlier than in the peripheral serum, resulting in a difference of 1.7 h in the time of maximum serum concentration. This indicates fast gastrointestinal absorption of FT but hepatic retention (without metabolism) before the appearance of FT in the peripheral serum.