Objective To establish a method for simultaneously measuring L-citrulline and L-arginine concentration in plasma using reversed-phase high performance liquid chromatography (RP-HPLC)with ultraviolet detection,and the novel method is applied to the patients with digestive system tumor.Methods Plasma samples were deproteinized by trichloroacetic acid with heating method.Phenylisothiocyanate (PITC)was used as derivatization reagent and a gradient elution was adopted. The feasibility verification of method was carried out by detecting plasma L-citrulline and L-arginine acid concentration in 21 cases of advanced digestive tract malignant tumor patients,21 cases of non malignant tumor patients and 39 cases of healthy people.Results The linearity for L-citrulline and L-arginine ranged from 0 to at least 1 000 μmol/L.r=0.999 95 for both. The lower limits of quantification for L-citrulline and L-arginine were 0.240 μmol/L and 0.448 μmol/L respectively.The in-traday and interday coefficients of variation (CVs)were less than 3.4% and 7.2%,respectively.The average recovery rate was from 95.2% to 104.1%.The ratio of plasma L-citrulline/L-arginine in patients with digestive cancar was significantly higher than that in the healthy control group,t-test,P =0.009.The sensitivity and specificity of plasma L-citrulline/L-argi-nine ratio in the malignant tumor of digestive system were 81% and 71.4% respectively.Conclusion This method can pro-vide a reliable and efficient method for the clinical determination of the L-citrulline and L-arginine concentration in plasma. The ratio of plasma citrulline/arginine increased obviously in patients with digestive cancer,suggesting that this ratio is a more sensitive index than single concentration of L-citrulline or L-arginine when evaluating patients with digestive tumor.