WHEN an aqueous solution of either iodoacetamide or N-ethyl maleimide, in suitable concentration, is applied topically to the mucosa of a canine Heiden-hain pouch half an hour before the injection of hist-amine, the hydrochloric acid secretory activity of the pouch is inhibited completely, though reversibly1. This inhibitory action may be presumed to involve a competitive inhibition of one or more enzymes within the parietal cells, though Davenport's assumption2 that these enzymes contain sulphydryl still awaits confirmation. Instead of the water-like solution of hydrochloric acid ordinarily obtained in response to histamine stimulation, the pouch now pours out a mucinous cell-free liquid of pH greater than 7, and sometimes even as high as 8. Flow of this viscous liquid continues for many hours after drainage of the iodoacetamide or N-ethyl maleimide solution from the pouch, whereas the acid-secretory response to subcutaneously administered histamine usually lasts for no more than 1–1½hr. Further evidence that this mucinous material bears no relation to the parietal secretion is given by the fact that the same response is obtained when the instillation of iodoacetamide or N-ethyl maleimide into the pouch is not followed by the subcutaneous administration of histamine.