A three day test for the evaluation of oral anti-estrogenic activity is described using 20-23-day-old female Swiss albino mice which are stimulated with subcutancously administered estrone. The test compound is administered simultaneously by gavage. The end-point is the degree of inhibition of estrogen produced uterine growth. The relative anti-estrogenic potencies of 25 steroids are reported on the basis of the minimum weight of compound, administered by gavage, needed to produce a statistically significant inhibition of the uterine stimulation produced by 0.4 μg. of estrone. Correlations of structure and activity may be summarized as follows: reduction of the Δ4-3-kcto group in ring A to the 5α-3-kcto caused either a decrease in activity or no net change, loss of carbon 19, with few exceptions, caused increased activity, and the introduction of 17α-alkyl groups resulted in increased anti-estrogenic activity in about onehalf the cases studied. The anti-estrogenic activity of steroids was, with only one or possibly two exceptions, greater by subcutaneous injection than by gavage. Other comparative figures arc given for relative potencies by subcutaneous injection and gavage.