Intestinal permeability has been assessed with three different classes of permeability probes, viz various sugar mixtures,51Cr-EDTA and poly(ethylene glycol). The former two methods are having increasing clinical applications in the screening and assessment of small intestinal damage and51Cr-EDTA is now the preferred probe for routine clinical use. Poly(ethylene glycol)s have numerous disadvantages and are not recommended. Probes may be used both in vitro and in vivo and have been applied to a wide variety of clinical problems. In particular, NSAID induced enteropathy, a major complication of the chronic administration of these widely-used drugs, was recognized for the first time with51Cr-EDTA permeability measurements. The cytoprotective role of various prostanoids was also clearly demonstrated using51Cr-EDTA. It is anticipated that measurement of intestinal permeability will play an increasing role in clinical and research investigation and in the monitoring of intestinal disease.