The morphdogical changes in mast cells, induced by horseradish peroxidase (HRP) and distilled water were studied, using light and electron microscopy. The mesenteries of rats sacrificed at 30 minutes; at 1, 2, 4 and 18 hours; and at 1, 2, 3 and 6 weeks after an intraperitoneal injection of 250 µg or 500 µg of HRP or 10 ml of distilled water were used for observation of degranulation rates and morphological changes in cell organelles during specified time intervals. The results were as follows: 1) A distinct increase in the mast cell degranulation was observed in the mesenteries after injection of HRP. In particular, all mast cells were disrupted or degranulated significantly 18 hours after injection of distilled water. 2) The degranulating mast cells of the group injected with HRP showed a progressive decrease in electron density of the granules, formation of perigranular cavities, and extrusion of the granules after partial loss of the plasma membrane. 3) At 2 weeks after injection of HRP, some indicators of the early stage of regeneration, that is, peripherally placed nucleus, the expanded Golgi area containing numerous vesicles, and progranules in these vesicles, were observed. 4) After 6 weeks, the morphological characteristics of most mast cells in the group injected with HRP were nearly the same as those of the control group. 5) In degranulating mast cells of the group injected with distilled water, partial rupture of the plasma and cytoplasm with wide dispersion of the granules.