The effect of hydroxypropyl starches (degree of substitution (DS) 0.025-0.106), lactose, raw potato starch, polyethylene glycol 1000 or magnesium sulphate on caecal size and some constituents of the caecal contents was studied in male rats of ages varying from 4 wk to 3 mth in experiments lasting from 10 days to 3 mth. All the test compounds induced caecal enlargement under the experimental conditions. Caeca enlarged by a hydroxypropyl starch (DS 0.047), lactose or raw potato starch, returned to normal sizes within 4 wk after the animals reverted to a control diet. The analytical data did not show a consistent relation between caecal size and the percentages of dry matter, sodium, potassium, chloride or volatile fatty acids in the caecal contents. The osmotic values of the caecal contents of control and experimental groups were of the same order of magnitude. It is postulated that the size of the rat caecum is controlled by the osmotic value of the caecal contents, irrespective of the nature or origin of the compounds contributing to this value and the conclusion is drawn that caecal enlargement is a process of physiological adaptation. Chemicals/CAS: lactose, 10039-26-6, 16984-38-6, 63-42-3, 64044-51-5; macrogol, 25322-68-3; magnesium sulfate, 7487-88-9; starch, 9005-25-8, 9005-84-9; Chlorides; Fatty Acids; Potassium, 7440-09-7; Sodium, 7440-23-5; Starch, 9005-25-8