It has previously been reported that exogenous adenosine-5’-triphosphate (ATP) evokes a marked histamine release from isolated rat mast cells with accompanying morphological changes including degranulation when Ca2+ is present in the medium (1, 2). Since it has been suggested that the energy requiring process is involved in the histamine release (3, 4), it seems to be reasonable to assume that ATP can be utilized as a source of energy supply in the process of histamine release induced by this compound, and that Ca2+ plays a role as a cofactor of ATPase of mast cell membrane. Diamant and Krüger (5, 6) and Diamant (7) who observed ATP-induced histamine release stated a possibility that Ca-activated “ecto-ATPase” in the mast cell membrane plays an important role in this histamine release.The present investigation was undertaken to see whether inorganic phosphate can be liberated from ATP when histamine release was induced by ATP from the mast cells in the presence of Ca2+ and whether mast cells are responsible for the 5’-nucleotidase activity in this respect, in order to know if the exogenous ATP really serves for the energy source in the histamine release.