A sensitivity to dietary cereal has been reported previously in niacin-deficient rats by measuring a change in the intestinal absorption of radioactively-labelled cellobiotol and mannitol. The possibility that other stimuli could produce this sensitivity, the range of cereals that could induce the permeability change and the nature of the toxic component in cereal have now all been investigated. Treatment with triparanol induces sensitivity in rats to wheat, rye, barley, oats, and maize but not to rice or soybean. These cereals caused a similar response in niacin-deficient rats. Mucosal damage produced by methotrexate or cetrimide, however, did not sensitise the intestinal mucosa to dietary cereals. Gluten, zein, and pepsin/trypsin digests of gluten all induced the permeability defect in triparanol-treated rats. It is concluded that although gross disruption of the mucosal structure may not sensitise rats to cereals, various causes of mucosal cell damage can produce a susceptibility to gluten toxicity that resembles gluten-sensitivity in man.