SUMMARY: Spayed mice drinking dilute aqueous solutions of oestrone (40 μg./1.) maintain a state of vaginal cornification for several weeks. A single injection of progesterone inhibits the vaginal response and the degree of inhibition can be measured. The ED 50 of progesterone by this method is about 75 μg. A satisfactory 3 + 3-point assay of megestrol acetate, using progesterone as a standard, is reported.