 The short length of the rat's estrous cycle makes them ideal for investigation, and a fast and practical method was developed to determine their estrous cycle phases by collecting vaginal secretion every morning for a month and observing unstained native material using a microscope without a condenser lens. Using the 10x objective lens, it is easier to analyze the proportion of cellular types present in the vaginal smear while the 40x objective lens helps recognize each cellular type. The determination of estrous cycle phases took only 15 to 20 minutes for all 30 female rats. This article was authored by F. K. Markons, F. J. B. I. A. N. C. H. I. and A. P. Tenno.