A new concept, “Radiation-Induced Drug (RID)”, was proposed as a novel type of drugs for cancer therapy. To test the usefulness of this concept, 5-fluorouracil (5-FU) derivatives having various types of substituents at the 1-position were prepared and the γ-radiolyses of their aqueous solutions were studied. The compounds having sulfonyl and thioureido groups as the substituents produced efficiently 5-FU with high G values upon γ-irradiation. The medium effects on the radiolyses revealed that the above two substituents were removed mainly with HO· and hydrated electrons eeq−.