Mast cells located within connective tissue are responsible for inflammation reaction and hypersensitivity in allergic responses. These cells synthesize and store histamine and heparin within cytoplasm. When provoked, they release these materials to cause inflammation and allergic responses. Mast cells also concern damage recovery of tissues, smooth contraction and vascular permeability. Cytoplasm contents such as serotonin, dopamine and protease as well as histamine and heparin, may mediate those respoa population of cells containing mast cell were stained with toluidine blue, After toluidine blue staining, mast cell granules were identified by further staining with alcian blue and safranin 0. All 3 types of uterine tissue mast cells, were categorized as connective tissue-type. The mast cells were red granules. mucosal-type mast cells were blue granules. and mixed-type mast cells were red and blue granules. Pregnancy caused drastic changes in uterine tissue. The fertilized eggs develop into early embryos and implant themselves around 4th day. Decidual tissue surrounds the implanted embryos during the early half of pregnancy. During early pregnancy, the number of mast cells decrease around implantation and increased later. Pseudopregnancy, however, did not caused such decrement around implantation. The discrepancy in mast cell density between pregnancy and pseudopregnancy was not due to the difference in hormonal profile. The concentration of estrogen did not show any significant difference between the two groups during early pregnancy. Only the presence of embryo in the uterus seemed to be responsible for this discrepancy.