On the basis of some assumptions concerning gastric emptying a theory of gastric function is developed which explains the known facts —chiefly the emptying of liquids according to an exponential law. On the basis of this theory an equation is derived which relates the rate of secretion to the quantity of liquid retained in the stomach after a certain time. A study is made of the general characteristics of this equation. Some assumptions concerning different formulae describing the rate of secretion are found useful in experimental investigations, particularly in Hollander’s dilution indicator method.