We examined the distribution of uterine mast cells in cycling mice of different ages between 7 weeks and 38 weeks by toluidine blue staining. Mast cell density increased gradually depending on the age and culminated at 30 weeks after birth. Of the estrous stages, uteruses at metestrus showed markedly higher density of mast cells than those at the other stages in cycling mice of all ages analysed in this study and the majority of the mast cells were found in myometrium. We also discriminated types of the uterine mast cells in cycling mice at 10 weeks old by Alcian blue-safranin double staining. Mucosal type was most abundant among three types, including connective-tissue types and mixed types, through the entire estrous cycles. The portion of mixed types increased slightly after estrous. The abundance of uterine collagen protein determined by Masson trichrome staining was exactly consistent with the density distribution of the uterine mast cells during estrous cycles. Taken together, these results may imply that uterine mucosal mast cells, together with collagen proteins, are heavily involved in tissue remodeling of cycling mouse uterus.