The concentrations of carmofur, 1-hexyl-carbamoyl-5-fluorouracil (HCFU), in the serum, bile, pancreatic juice and pancreatic tumor tissue were studied in 18 cases of peri-pancreatic head cancer with drainages of the pancreatic and biliary ducts after pancreatico-duodenectomy. As a result, high concentrations of HCFU and 5-FU were detected in the serum, bile and pancreatic juice after a per-oral administration of HCFU, 200 mg: HCFU was high in the order of serum > bile and pancreatic juice, and 5-FU in the order of bile, serum and pancreatic juice. Two hours after administration, 5-FU concentration in the bile and serum attained to the maximum levels of 0.45 and 0.19 micrograms/ml, respectively, which demonstrated a lasting transfer of 5-FU to the bile at high concentration. HCFU and 5-FU levels in the tumor tissue were 0.079 and 0.024 micrograms/g, respectively. In conclusion, antitumor effect against the malignant tumors of the pancreatobiliary system can be expected by peroral administration of carmofur.