The Rat Vaginal Impedance Checker is designed to obtain precise information to determine the optimum day for mating during the rat estrous cycle. The electrical impedance of the epithelial cell layer of vaginal mucosa is measured at the frequency of 1 kHz by insertion of the probe into the vagina. In the proestrus stage significantly higher impedance is produced compared to that in the other stages of the estrous cycle. 3 kohm of impedance can be considered a standard for indicating proestrus stage. Measuring range is 0 - 19.9kohm.The conventional method (vaginal smear method) is a troublesome procedure requiring skill, time and labor.