(1) The ‘true antiestrogens’ are estrogenic and are competitive inhibitors of estrogen action at the estrogen-sensitive target organs. The estrogenicity of a compound needs to be specified with reference to (a) the target tissue, (b) parameter used, (c) route of administration, (d) dose of the compound, (e) time sequence of action. The degree of antiestrogenicity is directly proportional to the, estrogenicity of the compound. The antiestrogenic action of a compound is related to its affinity with the receptor sites in the target organ: (a) compounds which are bound to the receptors for brief periods are weakly estrogenic (DMS, cis-clomiphene), (b) compounds which have greater binding affinity with the receptors in the target organ are strongly estrogenic and antiestrogenic (CN 55, 945–27; trans-clomiphene, DES, MER-25 at high doses). The duration of antiestrogenic action of a compound is related to the duration of its binding with the receptor in the target organ.