To evaluate the usefulness of 5'-deoxy-5-fluorouridine (5'-DFUR) therapy for brain tumor, the pharmacokinetics of 5'-DFUR and 5-fluorouracil (5-FU) in the tumor, in the white matter surrounding the tumor and in the serum were investigated, and the thymidine phosphorylase, the main activating enzyme of 5'-DFUR in human tissues, was also studied. Materials were obtained from 24 cases of primary brain tumors and from 4 cases of metastatic carcinoma. During the operation 500 or 1, 000 mg of 5'-DFUR was administrated intravenously, and blood, tumor tissue and white matter were taken mainly within 60 minutes and partly within 180 minutes after intravenous administration of 5'-DFUR. In some cases samplings were performed serially. Concentrations of 5'-DFUR and 5-FU were measured by high performance liquid chromatography, and thymidine phosphorylase activity in tissue extracts was obtained against d-thymidine and/or 5'-DFUR as substrates. The concentrations of 5'-DFUR and 5-FU in serum decreased immediately in an exponential mode after intravenous administration of 1, 000 or 500 mg of 5'-DFUR. High concentrations of 5'-DFUR and 5-FU in the tumor tissues were noticed in malignant tumors, such as glioblastoma, ependymoma and chondrosarcoma, and 5-FU concentrations in the tumor in two cases of glioblastoma and one of ependymoma were higher than that in their sera. An important characteristic of 5'-DFUR was that it was converted to 5-FU in various tissues predominantly in malignant neoplasms. Thymidine phosphorylase activity was more prominent in glioblastoma, metastatic carcinoma and chordoma. Although the 5'-DFUR concentration in white matter was considerable, the 5-FU concentration was negligible suggesting low enzyme activity. An effect of 5'-DFUR therapy on malignant brain tumor should be expected from the high concentration of 5' DFUR and 5-FU in the tumor tissue. However, these high concentrations rapidly decreased, therefore, the appropriate mode of prescription of 5'-DFUR should be considered in clinical practice.