A rapid, sensitive and reproducible ion-pair, reversed-phase high-performance liquid chromatographic (HPLC) method was developed to simultaneously quantitate 5-fluorouracil (5FU) and its prodrugs, tegafur (TF) and 4-deoxy-5-fluorouracil (DFU), in rat plasma. 5FU, TF, and internal standard 2', 3'-didehydro-2'-deoxythymidine (D4T, stavudine), were detected by UV absorption at 254 nm, and DFU was detected at 313 nm. Extraction recoveries for all compounds ranged between 80% and 92%. Retention times of 5FU, D4T, DFU and TF were 4.6, 5.5, 6.8 and 11.5 min, respectively. Calibration plots were linear over the range of 0.1 microgram/ml to 50 micrograms/ml for 5FU, and 0.25 microgram/ml to 50 micrograms/ml for TF and DFU. The limit of quantitation was 0.1 microgram/ml for 5FU and 0.25 microgram/ml for TF and DFU. The intra- and inter-day variations were less than 10% and accuracy was greater than 90%. This method was applied to plasma samples collected from rats that were administered 5FU, TF and DFU intravenously.