The effects of exogenous estradiol valerate on some uterine characteristics of the musk shrew were investigated. Treatment with the steroid for either 1 day or 7 days did not noticeably alter luminal epithelial cell height, endometrial gland epithelial cell height or diameter, or number of endometrial glands. The reciprocal values of cell density of circular muscle and deep endometrial layers and endometrium-to-myometrium ratio of the uterus increased significantly in response to 7 days of steroid administration. After 1 day of steroid treatment the numbers of mast cells in different layers of the uterus (i.e., meso-, myo-, and endometrium) were unchanged, but after 7 days there were significant increases in the number of mast cells in meso- and myometria. The number of eosinophils in all three layers of the uterus increased significantly in response to the treatment of estradiol for 1 day or for 7 days. The increase was greater in the 7-day group. Neither uterine DNA nor RNA contents changed following administration of the steroid for either group, although protein content was elevated significantly in the 7-day group. Estradiol administration thus evokes small but subtle changes in the uterus of the musk shrew.