In a first attempt to find out the estrogenic activity of the monthly injectable contraceptive composed of dihydroxyprogesterone acetophenide 150 mg + estradiol enantate 10 mg (Perlutal, Topasel), estradiol enantate (estra-1,3,5(10)-triene-3-ol-17 beta-heptanoat), estradiol benzoate and ethinylestradiol were compared in rats of the Chbb: THOM species. Estradiol benzoate potency was significantly higher than that of estradiol enantate: 20.34 (15.57-26.31) times in the vaginal cornification test in castrated females; 2.21 (1.63-2.98) times in the uterine weight test also in castrated rats and 2.91 (1.86-4.54) times in the vaginal opening test in immature rats. Ethinylestradiol, orally administered in the first two tests, was less active than the other two parenterally given substances; it overcame them in the vaginal opening test, when all substances were perivaginally administered. The duration of action of equivalent doses on the vaginal smear in castrated females resulted higher for estradiol enantate: 2.52 times more than estradiol benzoate and 5.2 times more than ethinylestradiol. Based on these results and on preexistent literature, the estradiol enantate dosis which would be sufficient to obtain the endometrial proliferation in women was extrapolated and compared with those already established of estradiol benzoate and ethinylestradiol. Finally, the possibility that the estrogenic potency of the injectable contraceptive treatment can be similar or lower than the oral one with pills containing ethinylestradiol 0.030-0.050 mg is discussed.