5-Fluorouracil (5-FU) metabolism on tumors was studied by in-vivo 19F-MRS (magnetic resonance spectroscopy). In this study, two kinds of tumors were used, i.e., Yoshida sarcoma implanted subcutaneously to the abdomen of rats and drug-induced tumors in the rats livers. Sequential 19F spectra were obtained just after 150 mg/kg 5-FU injected intravenously. In Yoshida sarcoma, the accumulation of 5-FU was observed and disappearance of 5-FU was slower compared to the normal tissue. However, synthesis of fluoronucleotides (Fnct) could not be detected. In drug-induced liver tumors, the peak of fluoro-beta-alanine (FBAL) was observed. Disappearance of 5-FU and catabolism to FBAL in the liver tumors group were slower compared to the normal liver. Synthesis of Fnct did not increase in the liver tumor group. The results in the liver tumor group are considered to be the confined result of the hepatocytes and tumors cells. It was considered that the delayed catabolism to FBAL in the liver tumor group showed metabolic dysfunction of the liver. Also the synthesis of Fnct in tumors could not be detected by in-vivo 19F-MRS. 19F-MRS method could not detect Fnct in tumors in-vivo. However, the accumulation of 5-FU could be assessed by this method. It is expected that the evaluation of 5-FU pooling in tumors could be used for the index of chemotherapeutic effect.