Rats were fed diets containing 0.5 mg or 50 mg of chrysotile asbestos each day for 1 week or 14 months and tissues of the gastrointestinal tract were examined by light and electron microscopy. At the light microscope level the oesophagus, stomach and caecum in treated animals appeared unaffected, whereas accumulation of cellular debris and Alcian blue-positive material was apparent in the ileal, rectal and colonic lumens. Electron microscope examination of the colon and ileum of rats ingesting 50 mg chrysotile/day for 14 months confirmed these findings and indicated changes in the mucosal lining cells of the ileum which were consistent with a mineral-induced cytotoxicity. These results are compared with those reported in a similar previous biochemical study and the specificity of asbestos action on certain regions of the gastrointestinal tract is discussed.