1) Sixteen normal healthy subjects free from occult blood in the stool were selected and administered with their 51Cr labeled own blood via duodenal tube and the recovery rate of radioactivity in feces and urine was measured. The average fecal recovery rate was 90.7 percent (85.7∼97.7%) of the administered radioactivity, and the average urinary excretion rate was 0.8 percent (0.5∼1.5%). 2) There was a close correlation between the amount of blood administered and the recovery rate from the feces; the more the blood administered, the higher the recovery rate was. It was also found that the administration of the tagged blood in the amount exceeding 15ml was suitable for measuring the radioactivity in the stools. 3) In five normal healthy subject, whose circulating erythrocytes had been tagged with 51Cr, there was little fecal excretion of radioactivity (average 0.9 ml of blood per day). This excretion is not related to hemorrhage and the main route of excretion of such an negligible radioactivity was postulated as gastric juice and bile. 4) A comparison of the radioactivity in the blood and feces of the patients with 51Cr labeled erythrocytes seems to be a valid way of estimating intestinal blood loss.