Intraperitoneal injection of distilled water caused osmotic disruption of the mast cells and the appearance of an appreciable amount of histamine in the peritoneal fluid.

Injection of Tyrode solution alone was not particularly damaging to the mast cells and little or no histamine was released.

Injection of Tyrode solution containing compound 48/80 resulted in extensive release of granules from mast cells and the appearance of large amounts of histamine in the fluid. Solution of 48/80 failed however to cause histamine release when injected into rats whose subserosal mast cells had previously been destroyed.

A series of increasing doses of compound 48/80 had a graded morphological effect upon mast cells and resulted in a graded increase in the amount of histamine that appeared in the peritoneal fluid. It is unlikely therefore that this compound acts by simply lysing the plasma membrane.

It is concluded that mast cells in the rat are extraordinarily rich in histamine which is liberated under conditions which cause mast cells to release their granules. The histamine set free by the potent histamine liberator, compound 48/80, appears to come principally from the tissue mast cells.