Rate of excretion and metabolites in urine and feces of mice after intravenous or oral administration of ¹⁴C-cyclocytidine and ¹⁴C-aracytidine were examined. Furthermore, absorption from gastrointestinal tract was examined. After intravenous injection, about 80% of cyclocytidine was excreted in urine within 24 hours and not in feces. Main radioactive compound in urine was intact cyclocytidine and small amount of aracytidine and arauridine were detected, while two-thirds of ¹⁴C-aracytidine administered were excreted after metabolized to arauridine. A half of cyclocytidine was excreted in feces as intact when administered orally. Very small amount of cyclocytidine was excreted in expired air as CO₂. However, one-fourth of radioactivity was excreted in expired air after oral administration of ¹⁴C-aracytidine. Cyclocytidine was mainly absorbed from stomach and slightly from large intestine.