The transactivation properties of the two estrogen receptors, ER alpha and ER beta, were examined with different ligands in the context of an estrogen response element and an AP1 element. ER alpha and ER beta were shown to signal in opposite ways when complexed with the natural hormone estradiol from an AP1 site : with ER alpha, 17 beta-estradiol activated transcription, whereas with ER beta, 17 beta-estradiol inhibited transcription, Moreover, the antiestrogens tamoxifen, raloxifene, and imperial Chemical industries 164384 were potent transcriptional activators with ER beta at an AP1 site. Thus, the two ERs signal in different ways depending on ligand and response element, This suggests that ER alpha and ER beta may play different roles in gene regulation.