SummaryFaecal blood loss was measured in normal male volunteers using51Cr-labelled red cells. In a double-blind parallel study in 10 subjects, the effect of 250 mg diflunisal twice daily was compared with 750 mg aspirin 4-times daily. Drugs were taken during two 7-day periods separated by a 1-week control period. Mean daily faecal blood loss during the two treatment periods was 0.32 ml and 0.53 ml in the diflunisal group versus 6.87 ml and 3.20 ml in the aspirin group. Diflunisal did not significantly increase blood loss, while aspirin had a significant effect. In a double-blind crossover study in 12 subjects, the effect of 250 mg diflunisal twice daily was compared with 600 mg aspirin 4-times daily. Alcohol (120 ml, 40%) was added during the last 2 days of each 6-day treatment period. Faecal blood loss was not significantly affected by diflunisal and there was also no significant effect on blood loss when alcohol was co-administered. Aspirin significantly increased faecal blood loss and this effect was significantly enhanced by the addition of alcohol.