Mucus is secreted throughout the gastrointestinal tract. The primary secretion is the water insoluble gel adherent to the mucosal surface but substantial amounts of mucus also occur in the lumen. The following are discussed: the functions of mucus; the structure and properties of mucus; study of the adherent mucus gel on the mucosal surface; the effects on mucus properties of proteolysis; thiol agents; bile salts; acid and hyperosmolar solutions. Much of our work on mucus has been with that from the stomach but where possible studies on the intestines and colon are discussed. Studies show that mucous secretions from the different regions of the gastrointestinal tract have similar rheological properties although the component glycoproteins differ in their detailed structure.