Mice bearing the murine colon carcinomas Colon 38 or Colon 26 were treated with 5-fluorouracil (5 FU) (60 or 100 mg/kg, respectively) weekly for 4 weeks at 08.30 hr or at 18.30 hr. The antitumor effect of treatment at 08.30 hr was significantly better for both types of tumors, but most pronounced for Colon 38. Toxicity was evaluated after administration of 60 mg 5 FU/kg. No thrombocytopenia was observed. Leucopenia was observed after treatment at 18.30 hr. It is concluded that the therapeutic efficacy of 5 FU against murine colon cancer is higher when 5 FU is administered at 08.30 hr.