An important function of the gastric mucosa is to contain within the lumen of the stomach the acid which the mucosa itself secretes. This function is expressed as the very low permeability of the mucosa to hydrogen ions. The interstitial fluid of the gastric mucosa contains sodium ions at a concentration very nearly equal to the sodium concentration of plasma, and the same property of impermeability which prevents hydrogen ions from diffusing into the lumen prevents sodium ions from diffusing from the mucosa into the lumen (1). Capillaries of the gastric mucosa, like most other capillaries, are highly permeable to electrolytes and to small molecules of the plasma. Their permeability to plasma proteins has not been measured, and this becomes an important question when one learns how much plasma protein is shed into the gastric lumen when the mucosa is damaged. The classical means of estimating the protein concentration of interstitial fluid of an organ is to measure the protein concentration of the lymph collected from that organ. Until recently lymph has not been collected from the stomach for the reason that the arborization of lymphatic vessels draining the stomach makes cannulation difficult or impossible. Dr. Teresa Bruggeman working in my laboratory has, with great pertinacity, applied the methods of renal micropuncture to collection of gastric lymph (2). She has found that the albumin concentration of gastric lymph is about 80% of that of simultaneously collected plasma samples. Gastric lymph also contains the other plasma proteins, including fibrinogen. If in fact gastric lymph concentration of albumin does equal interstitial concentration, the high concentra