[Comparative action of an impeded estrogen, zeranol, and estradiol on some receptors in the female rat (author's transl)].

We have studied the estrogenic action of zeranol and have compared it to the action of estradiol on the uterus--the vagina, the mammary gland and the pituitary gland of the pre-pubescent, castrated female before the opening of the vagina. The weakest dose at which estradiol is active is 0.1 microgram injected for a period of three days : it is capable of opening the vagina on the fourth or fifth day and of developping the uterine glands to the maximum : injected over a period of ten days it provokes a hypertrophy of the pituitary gland : only the mammary gland reacts exclusively to doses other than physiological ones. The estrogenic action of zeranol is weak : 1 mg administered orally for three days is necessary in order to reproduce the same effects of 0,1 microgram of estradiol on the opening and the keratinisation of the vagina and on the stimulation of the uterine epithelium ; its efficacy is greater on the uterine glands (250 micrograms is equivalent to 0.1 microgram) and on the pituitary gland (500 microgram reproduce the hypertrophy provoked by 0,1 microgram). The doses administered are by 100 g of body weight. The uterine glands are more sensitive to stimulation by the estrogens employed (estradiol, zeranol) than is the uterine epithelium. The study of the relation between the estrogenic dose and the response of the uterine weight shows that the gradient of the curve is weaker with zeranol than with estradiol. This observation shows that zeranol behaves as an impeded estrogen.