It is well established that the gonadal steroid estradiol-17-β plays a major role in the regulation of female sexual behavior (ie, lordosis) by acting through intracellular estrogen receptors, ligand-dependent transcription factors. Two very similar nuclear receptors, ER-α and ER-β, which are probably gene duplication products, must be examined. Recent studies using gene knockout mice for estrogen receptors, either ER-α or ER-β, have advanced our knowledge about how estrogen receptors are involved in the control of this behavior and have further revealed that they play a role in the regulation of an array of behaviors related to reproduction in both sexes. These studies suggest that activation of ER-α and ER-β may work either synergistically or antagonistically, depending on the behavior, to fine-tune the final outcome of the behavior. Patterns and combinations of ER-α and ER-β expression modulate patterns of social behaviors. Further studies to determine the molecular and physiologic mechanisms downstream to transcriptional actions of estrogen receptors will deepen our understanding of estrogenic regulation of a range of social behaviors.