The acute effect of three non-steroidal anti-inflammatory drugs, ibuprofen, acetylsalicylic acid (ASA) and indoprofen, on faecal blood loss was investigated in 15 subjects by means of 51Cr-labelled erythrocytes. Ibuprofen (900 mg/day for 5 days) and indoprofen capsules and tablets (300 mg and 600 mg/day for 5 days, respectively) slightly increased the amount of blood eliminated in faeces. The increase was of the same order of magnitude for both doses of indoprofen. ASA (1,500 mg/day for 5 days) caused about a 6-fold increase in blood loss. Four days after withdrawal of ASA, faecal blood was still about twice as high as in faeces of subjects given ibuprofen and indoprofen. The method appears sensitive and reliable for comparison of the immediate effect of anti-inflammatory drugs on gastro-intestinal mucosa.