Gas chromatography-mass spectrometry (GC/MS) and high performance liquid chromatography (HPLC) procedures have developed to determine 5-fluorouracil (5-FU) in human plasmas. Internal standard in both systems was 5-bromouracil (5-BrU). The HPLC assay used an extracted plasma with an ether-isopropanol mixture (75-25): the analyses were performed on a mu Porasil column (10 micrometer) with hexane-ethanol (75-25) as elution solvent (detection limit 50 ng/ml). For GC/MS, after precipitation of proteins, the plasma was purified through a microcolumn packed with AG1 X 4 resin, and then the components were methylated. Electron impact (EI) and selective ion monitoring (SIM) mode was retained for GC/MS quantitative studies (detection limit 5 ng/ml). Moreover, the authors compared these two techniques by measurements on plasma samples obtained from patients after rapid i.v. injections, infusions and p.o. administration.