Determining the Stage of the Estrous Cycle in the Mouse by the Appearance of the Vagina

A simple method is described by which the various stages of the estrous cycle of the mouse can be easily determined by the appearance of the vagina. Data are presented which show that this method of determining estrous state is at least as accurate and reliable as the customary method of vaginal smearing. The method is faster, and less stressful to the animals than the vaginal smear procedure.