The colon is subjected to a myriad of potentially damaging agents that may reside within the lumen for 1-2 d. Its first line of defence against these agents is the protective mucus bilayer that lines the entire colonic mucosa. This bilayer acts as a physical barrier to mucosal aggressors and also reduces shear stress to the mucosa. These actions are dependent on the unstimulated ('resting') colonic mucus thickness, and also on the rate that this layer can be replenished. The colonic mucus layer is altered in a number of colonic diseases that have been linked to a deficiency of fibre in the diet. The action of fibre intake on colonic mucus thickness and secretion is unknown. Using an in vivo rat model it has been demonstrated that: (1). fibre deficiency leads to a decreased protective potential of the mucus layer (e.g. the mean resting mucus thickness of the fibre-deficient group (429 microm) was significantly lower than its respective control (579 microm; P< 0-001), as was its total mucus secretion over 6 h (270 microm v. 541 microm; P<0-01); (2). specific fibre types in the diet alter the secretion dynamics of colonic mucus (e.g. a cellulose-based diet reduces total mucus secretion over 6 h compared with its control (175 microm v. 463 microm). Analysis of the diets suggested a necessity for both soluble and insoluble fibre types in the diet to increase mucosal protection.