Type I allergic reaction and it s related clinical manifestations are known to occur by the effects of various chemical mediators. These chemical mediators are released from circulating basophils and tissue mast cells, which become sensitized through the binding of antigens and antibodies of the IgE type to their cell surface receptors. Efforts to elucidate the mechanism of the release of these mediators, especially that of histamine, have been persued for years. The mechanism is not yet clarified at the present time. Recent reports of hyaluronidase, an enzyme known to be involved in the tissue inflammatory process, as possible participant in type I allergic reaction, initiated this study. Relationships between the hyaluronidase activity and histamine release from the sensitized rat peritoneal mast cells were investigated. Also anti-allergic agents, tranilast and disodium cromoglycate, along with known histamine releasers, morphine and compound 48/80, were used to observe the inhibitory and stimulatory effects of these substances on the hyaluronidase activity as well as histamine release from the rat mast cells. The results obtained are summarized as follows: 1) Hyaluronidase activity and histamine release from sensitiaed rat peritoneal mast cells started to increase on the 4th day of postsensitization. Hyaluronidase activity reached it s peak value on the 7th day of postsensitization and that of histamine release on the 14th day of postsensitization. 2) Hyaluronidase activity and histamine release from sensitized rat peritoneal mast cells, pre-treated with tranilast revealed significant decrease in comparison with those of non-treated cells. 3) Hyaluronidase activity and histamine release from sensitized rat peritoneal mast cells, pre-treated with tranilast, followed by morphine injection, revealed significant increase in comparison with those of tranilast treated cells. 4) In vitro study of hyaluronidase activity and histamine release from un-sensitized rat peritoneal mast cells, using morphine and compound 48/80 as activators, revealed significant increase compared to those of non-activator used cells. 5) In vitro study of hyaluronidase activity and histamine release from un-sensitized rat peritoneal mast cells, pre-treated with tranilast and disodium cromoglycate, using confound 48/80 and morphine as activators revealed significant decrease in comparison with those of tranilast and disodium cromoglycate treated cells. From above results, participation of enzyme hyaluronidase in the process of histamine release from sensitized rat pertioneal mast cells, could be suggested. It was also quite evident that the clinically used anti-allergic agents, tranilast and disodium cromoglycate, have significant inhibitory function on the hyaluronidase activity and histamine release from sensitized rat peritoneal mast cells, while morphine significantly increased the hyaluronidase activity and histamine release from sensitized rat peritoneal mast cells.