Forty patients with gastric cancer and 30 patients with colon cancer were administered FT-207 prior to the operation. The relation between a total dose (4-88g) and tissue concentration of 5-FU was investigated and the following results were obtained. (5-FU concentration was measured with the method of Gas chromatography mas fragmentography) 1) The relation between total doses of FT-207 (x) and concentration of 5-FU in the tissue (y) was demonstrated by the formula: y = 0.00317x + 0.025 (gastric cancer) r = 0.519, p = 0.0001 y = 0.0019x + 0.043 (colon cancer) r = 0.641, p = 0.0001 2) In most of the patients with rectal cancer who received radiation therapy prior to the operation, 5-FU concentration in the tissue was extremely low. 3) 5-FU concentration showed no difference between the normal and metastatic lymph nodes, or among the lymph node groups. Administration methods (oral intake or suppository) had no influence on the concentration of 5-FU. 4) Among the organs, 5-FU concentration was higher in the following orders: liver, normal mucosa, lymph node, tumor, normal serosa. 5) In five autopsy cases, 5-FU concentration of the tissue show a great difference among them, and some cases showed 10 times as high concentration as others in the tissue of every organ.