Mast cells of the skin and omentum of mice, and of the omentum, mesenterium and subcutaneous tissues of rats were fixed and stained with a modified method of Zahl-Nowak's (1949) and with Riley's (1953) method, taking care to avoid artefact as much as possible. For statistical analysis of the disruptive changes of the cells after the drug administration, the morphologic changes were divided into three or four grades and the percentage was determined of those cells underwenting morphologic change. The disruption by artefact could easily be distinguished. After intravenous or subcutaneous injection of a definite quantity of sinomenine, a histamine liberator (Mayeda. 1953), disruption of various grades occurred and metachromatic change or decoloration of the granules was also observable in the mast cells of the skin of mice and of the omentum and mesenterium of rats. Statistically this disruption was highly significant. Such morphologic changes occurred equally both in what Riley (1953) calls Type-Ⅰ and Type-Ⅱ mast cells. It required a certain length of time before mast cells reached the maximum disruption after the drug administration. In the omentum of mice and in the subcutaneous tissues of rats the mast-cell disruption by sinomeniae was statistically not significant. This may be partly due to relatively more artefact in the preparation of these tissues. Previous injection of neoantergan partially inhibited the disruption of mast cells by sinomenine. Quinine and lycorine disrupted the mast cells of mouse skin just as sinomenine, while emetine even in the lethal dose showed no effect. The author discussed on the mechanism of the mast-cell disruption and histamine release.