Oral administration of uracil plus ftorafur [1-(tetrahydro-2-furanyl)-5-fluorouracil] has a greater antitumor effect than that of ftorafur alone. A high-pressure liquid chromatographic methods was developed for the determination of ftorafur in plasma and visceral tissues with a sensitivity of 0.025 microgram/ml or g of wet weight. A combination of GLC--mass fragmentography and GLC--mass spectrometry with total-ion monitoring was developed for the specific, simultaneous determination of 5-fluorouracil, and active metabolite, and uracil as their trimethylsilylated derivatives. The detection limits for 5-fluorouracil and uracil in the first method were 0.001 microgram/ml for plasma and 0.001--0.005 microgram/ml for visceral tissues, and those in the second method were 0.2 microgram/ml or g of wet weight. The precision and sensitivity of the assay appear to be satisfactory for determination of the levels of these compounds in plasma and visceral tissues.