AIM:To determine the expression levels of three metabolicenzymes of fluoropyrimidines:thymidylate synthase (TS),thymidine phosphorylase (TP) and dihydropyrimidinedehydrogenase (DPD) in seven human gastrointestinalcancer cell lines,and to compare the enzyme levels withthe sensitivity to 5-fluorouracil (5-FU) and 5-fluoro-2’-deoxyuridine (FdUrd).METHODS:TS,TP and DPD mRNA levels were assessedby semi-quantitative RT-PCR,TP and DPD protein contentswere measured by ELISA.Fifty percent inhibitoryconcentrations of growth (IC50),representing the sensitivityto drugs,were determined by MTT assay.RESULTS:IC50 values ranged from 1.28 to 12.26 uM for5-FU,and from 5.02 to 24.21 uM for FdUrd,respectively.Cell lines with lower DPD mRNA and protein levels tendedto be more sensitive to 5-FU (P<0.05),but neither IS norTP correlated with 5-FU IC50 (P>0.05).Only TS mRNA levelwas sharply related with FdUrd sensitivity (P<0.05),but TPand DPD were not (P>0.05).A correlation was foundbetween mRNA and protein levels of DPD (P<0.05),but notTP (P<0.05).CONCLUSION:DPD and TS enzyme levels may be usefulindicators in predicting the antitumor activity of 5-FU orFdUrd,respectively. AIM: To determine the expression levels of three metabolic enzymes of fluoropyrimidines: thymidylate synthase (TS), thymidine phosphorylase (TP) and dihydropyrimidine dehydrogenase (DPD) in seven human gastrointestinal cancer cell lines, and to compare the enzyme levels with the sensitivity to 5- fluorouracil -FU) and 5-fluoro-2’-deoxyuridine (FdUrd) .METHODS: TS, TP and DPD mRNA levels were assessed by semi-quantitative RT-PCR, TP and DPD protein contents were measured by ELISA. Fifty percent inhibitory concentrations of growth (IC 50 ), the sensitivity of the drugs, were determined by MTT assay .RESULTS: IC50 values ​​ranged from 1.28 to 12.26 uM for 5-FU, and from 5.02 to 24.21 uM for FdUrd, respectively. Cell lines with lower DPD mRNA and protein levels tended to be more sensitive to 5-FU (P <0.05), but neither IS nor TPPS with 5-FU IC50 (P> 0.05) .Only TS mRNA level was significantly related to FdUrd sensitivity (P < 0.05). A correlation was found between mRNA and protein l evels of DPD (P <0.05), but notTP (P <0.05) .CONCLUSION: DPD and TS enzyme levels may be usefulindicators in predicting the antitumor activity of 5-FU orFdUrd, respectively.