Effects of degradable starch microspheres administered via the hepatic artery were examined in rats in which an adenocarcinoma was transplanted into the liver. 3H-uridine or 3H-uracil with cold uridine and uracil, respectively, in amounts corresponding to therapeutic doses of these two pyrimidines as fluoro compounds, were administered with or without microspheres. Labeling of the acid-soluble fraction and RNA of tumor, liver, small intestine, spleen, kidney, and bone marrow was examined after 3 and 60 min after injection. When microspheres were added, the specific radioactivity of tumor RNA was significantly higher at both 3 min (P less than 0.05) and 60 min (P less than 0.01) in the rats given uridine, and in rats given uracil it was higher at 60 min after injection (P less than 0.05). There were no such differences in the labeling of the normal tissues. The results indicate that arterial administration of cytostatic drugs, such as 5-fluoropyrimidines, together with degradable starch microspheres might increase the cytotoxic effect on tumors nourished by the artery.