Relative Potencies of 17αand 17β-Estradiol in the Rabbit

The relative potencies of 17αand 17β-estradiol were compared as to their respective abilities to maintain luteal function in 25 mature, hypophysectomized rabbits and to promote uterine growth in 28 immature rabbits which had been ovariectomized between 8 and 10 weeks of age. Estradiol-17β was found to be 100 times more active in sustaining luteal function and at least 10 times more effective in promoting uterine growth than the α isomer. Luteal function was sustained in adult hypophysectomized animals with either 1 or 2 µg estradiol-17β (0.25 or 0.5 µg/kg) administered twice daily sc, and uterine growth and water uptake were stimulated in immature ovariectomized rabbits after 4 days of treatment during which 0.1 µg/kg of this estrogen was administered twice daily. These results suggested that, although both the uterus and the ovary are estrogen target tissues, less estrogen is required to promote uterine growth than to maintain lutal weight and activity. It is concluded that 17α-estradiol is not the principal estrogenic hormone of the rabbit.