Further report on the endocrinological profile of two synthetic estrogens with antifertility properties, STS 456 and STS 593.

Two synthetic estrogens, STS 456 and STS 593, with noteworthy post-coital antifertility properties in the rat were studied for their estrogenic, antiestrogenic, progestagenic, antiprogestagenic, antigonadotrophic, androgenic and antiandrogenic effects in laboratory animals. Beside their weak estrogenic activity which corresponds to their slight affinity to the uterine estrogen receptor in the rat, both steroids show remarkable antiestrogenic effects. No progestagenic, androgenic and antiandrogenic effects were found but STS 593 showed some antiprogestagenic activity. The antigonadotrophic efficacy of STS 456 was relatively high while STS 593 was scarcely active. The results are discussed with respect to possible application of these drugs for interceptive purposes.