18F-labeled 5-fluorouracil(FUra), 5-fluoro-2'-deoxyuridine(FdUrd), and 5-fluorouridine ( FUrd ) were synthesized with high radiochemical purities. The 18F-labeled pyrimidines were injected into rats. The metabolites in serum, bile, and urine were analyzed up to 2 h after administration by radio-high performance liquid chromatography (HPLC). The blood clearance of three pyrimidines was very rapid. In the serum the nucleosides and base disappeared very rapidly with a biological half-life of about 2 min and most of them had disappeared by 60 min. The metabolites in the urine were similar to those in the serum. In the bile pyrimidine nucleosides and base were not detected. 18F- was found in the metabolites. Our results explain the high uptakes in the kidney and liver in biodistribution studies of the 18F-labeled pyrimidines.