 The short length of the rat's estrus cycle makes them ideal for investigating changes during their reproductive cycle, and a fast and practical method was developed to determine the phases of the estrus cycle using unstained native material observed, under a microscope with 10x and 40x objective lenses, which took only 15 to 20 minutes to collect, observe, and determine the phase for all 30 female rats. This article was authored by F. K. Markons, F. J. B. I. N. C. H. I. and A. P. Tano.