Introduction. Recently, biological electrical impedance (EI) has been widely used as a method to assess body constituents. There are two methods for EI measurement: two-electrode (TEM) and four-electrode (FEM) methods. The apparatus for the TEM is generally simple and inexpensive comparing with the FEM, but the accuracy of measurement in the TEM is lower than in the FEM. The FEM involves two pairs of electrodes, an outer pair and an inner pair. Electrical current passes between the outer pair (current electrode), and the voltage between the inner pair (voltage electrode) is measured. The uterine endometrium (UE) changes its conditions chemically and histologically in each menstrual phase. Since EI varies depending on tissue conditions, EI of UE (UEI) may also vary according to menstrual phases. However, to the best of our knowledge, UEI has been rarely reported. The aim of this study was to clarify the characteristics of UEI using mice. Methods. Animals. Twenty 10-week-old Slc:ddy female mice were used in this study. We checked the estrus cycle of mice by cytological evaluations of vaginal smears using Giemsa staining. The cycle of the mice used consisted of proestrus (PES, n=4), estrous (ES, n=7), metestrus (MES, n=5), and diestrus (DES, n=4). Equipment. We used an impedance checker using the FEM (MLT-50, Toray Medical, Japan). The MLT-50 measures impedances at 140 points between 2.5 and 350 kHz automatically and calculates characteristic parameters such as the impedance at 0 Hz (R0) and infinity Hz (Ri), critical frequency (fc), and resistance (Rfc) and reactance (Ifc) at fc. The intrauterine probe (IUP) was composed of two electrodes and a core of tungsten wire of 0.3 mm in diameter. We rolled platinum wire on the core wire as voltage electrodes at two sites: 2.5 – 5.0 and 7.5 – 10.0 mm from the tip end. We pricked two thin needles into the skins of the fore- and hind-feet and used them as current electrodes. Procedure. Under anesthesia with continuously inspired isoflurane, we performed laparotomy in a supine position, and exposed the uterus. We inserted the IUP into the vagina and advanced its tip into the uterine cavity, and then measured UEI. Results. Our preliminary experiments revealed that there was little difference in values of UEI at the points where the tip of the IUP was located 15, 30, and 45 mm from the external uterine os. UEI at 1 kHz at ES (3.5 ± 0.3 kΩ, mean ± SEM) was significantly larger (p<0.05, Kruskal-Wallis test) than those at PES (1.6 ± 0.2 kΩ), MES (0.6 ± 0.1 kΩ), and DES (0.9 ± 0.1 kΩ). There were no significant differences in R0, Ri, fc, Rfc, and Ifc regardless of menstrual stages. Discussion and Conclusion. We measured UEI at various estrous stages using the FEM successfully. The UEI parameters measured by the FEM might be new parameters for evaluating UE conditions.