Contraceptive action of impeding estrogens VIII. The post-coital activity of estriol ethers in the rat

Evidence is presented to show that Estriol-3-cyclopentylether when administered in a single oral dose of 125 ug, post-coitum, completely prevents pregnancy in rats. Larger doses were shown to offer protection for more than one estrus cycle. Administration of 3 mg estriol-3-cyclo-pentylether even three days before mating still prevented pregnancies. Comparison of estrogenic potencies showed no significant differences between estriol and its cyclopentylether on the immature rat uterus.