51Cr-EDTA activity was measured in urine and blood of patients receiving non-steroidal anti-inflammatory (NSA) drug treatment and of healthy subjects and other patients (controls), after oral intake of 51Cr-EDTA, for the purpose of deciding whether NSA treatment increases urinary excretion of oral 51Cr-EDTA as an expression of increased intestinal permeability. 51Cr-EDTA activity in urine and blood of patients with rheumatoid arthritis (13) being treated with NSA was significantly higher (similar to results in 13 patients with Crohn's disease) than that of a control group (14) of patients with rheumatoid arthritis without such treatment (9) and patients without rheumatic disease (5). Both in patients with rheumatoid arthritis receiving NSA drugs and patients with Crohn's disease there was a highly significant correlation between urinary and blood activity. There was no effect of NSA drugs on renal function. The results indicate that NSA drugs increase interenterocytic permeability to an extent comparable to permeability abnormalities in Crohn's disease.