The stomach secretes a number of substances, viz: (1)hydrochloric acid; (2) proteolytic enzymes - the pepsin group and gelatinase; (3) the hormone - gastrin; (4) intrinsic factor; (5) mucus; (6) blood group substances; and (7) a variety of anions and cations. The production and secretion of (1) to (5) are controlled according to the requirements of the body. This programme attempts to explain how these substances are produced and the mechanisms which control their secretion.