Estradiol, estradiol-16alpha, estriol and a series of positional isomers of estriol were tested for their post-coital contraceptive activity, and their ability to compete with estradiol for the cytosolic estrogen receptor protein of rat uteri. Estrogenicity was determined for each compound in immature rats with single injections and in mature castrated rats by injecting the lowest fully contraceptive dose for four consecutive days. All compounds were active anti-implantational agents, varying in required dosage from 4 mug to 2000 mug (total dosage over 4 days). The rank order or contraceptive activity was found to be: estradiol greater than estriol = epiestriol greater than 11beta - (OH) - estradiol greater than estradiol - 16alpha greater than 7alpha-(OH)-estradiol greater than 6beta-(OH)-estradiol, while the order of estrogenicity at levels of contraceptive activity in the ovariectomized mature rat was: epiestriol = estriol less than 7 alpha - (OH) - estradiol less than estradiol - 16alpha less than 6beta - (OH) - estradiol = 11beta - (OH)- estradiol less than estradiol-17beta. Reasonably good correlation between competition for estrogen receptor and anti-implantational activity was observed. The most active binding competitors were estradiol-16alpha, epiestriol, and estriol which showed an affinity from 50-20% that of estradiol, while the other compounds had 3% or less binding competition for estradiol.