The role which the human colon fulfils in digestion and metabolism remains largely undocumented. Its capacity to conserve water and electrolytes is well known although how this is controlled is uncertain. In the animal kingdom, calcium and magnesium absorption from the colon are improtant as are absorption and synthesis of vitamins. The abundant microflora of the human colon gives it unique properties. Dietary residue is metabolised forming short-chain fatty acids, hydrogen, carbon dioxide and methane; whilst 20% of urea synthesised in man is broken down in the colon to ammonia, which is reabsorbed, and carbonic acid. The microflora also degrades a wide variety of organic compounds including food additives, drugs, bile salts, and cholesterol which may be relevant to the development of colon cancer. Regional differences in colonic function also exist making interpretation of data from this relatively inaccessible organ more difficult.