A new approach to the measurement of intestinal permeability using low molecular weight polyethylene glycols (PEG 400) has been applied to the study of normal and abnormal permeability states in man and animals. Successful assessments of gastric, jejunal, ileal, and colonic permeability suggest that the technique has application to any area of the gastrointestinal tract. The method has shown that bile acids alter mucosal permeability in the stomach and colon and that celiac sprue is associated with decreased intestinal permeability. These examples illustrate the potential of the technique for studying the function of diseases intestine in man.