A validated, sensitive and precise reversed-phase high-performance liquid chromatographic method for the simultaneous determination of 5-flucytosine (5-FC) and 5-fluorouracil (5-FU) in human plasma is described. Two compounds, 5-methylcytosine (5-MC) and 5-chlorouracil (5-CU), were used as internal standards for the determination of 5-FC and 5-FU, respectively. Plasma samples were deproteinized with trichloroacetic acid and chromatographed on an octylsilica column, maintained at 30 degrees C during elution, using a 0.04 M phosphate buffer, pH 7.0, as eleunt. Spectrophotometric diode array detection was used at 266 nm. 5-FC, 5-FU, 5-MC and 5-CU were found to have retention times of 4.8, 5.8, 7.7 and 11.0 min respectively. Recoveries of 91-120% with reproducibility and repeatability coefficients of variation of 0.8-6% were obtained. Mean correlation coefficients of 0.99989 and 0.9995 were found for the linear calibration curves (n = 2) of 5-FC (4.816-192.6 mg/l) and 5-FU (0.05368-5.368 mg/l), respectively. The limits of quantitation were 0.3 mg/l for 5-FC and 0.05 mg/l for 5-FU.