Transgenic mice expressing fluorescent proteins in specific cell populations are widely used for the study of in vivo behavior of these cells. We have recently reported that uterine mast cells (uMCs) are important for implantation and placentation. However, their in vivo localization in uterus before and during pregnancy is unknown. Herein, we report the direct observation of uMCs in vivo using doubletransgenic C57BL/6J Mcpt5-Cre ROSA26-EYFP mice with high expression of enhanced yellow fluorescent protein in MC protease 5 (Cma1 (Mcpt5))-expressing cells by intravital two-photon microscopy. We were able to monitor MCs live in utero during the murine estrous cycle and at different days of pregnancy. We demonstrated that uMCs accumulated during the receptive phase of the female (estrus) and persisted in large numbers at early pregnancy stages and around mid-gestation and declined in number in non-pregnant animals at diestrus. This intravital microscopy technique, including a custom-made microscope stage and the adaption of the surgical procedure, allowed the access of the uterus and implantations for imaging. The introduced application of intravital microscopy to C57BL/6J-Mcpt5-Cre ROSA26-EYFP mice offers a novel and powerful in vivo approach to further address the evident relevance of uMCs to reproductive processes with obvious clinical implications.