CRH and estrogens, produced by placental trophoblasts, have been suggested to play pivotal roles in the control of human parturition. Estrogen has been shown to affect hypothalamic CRH expression. Therefore, we evaluated 17 beta-estradiol (E2) in the regulation of CRH gene expression in placental cells. E2 inhibited CRH mRNA expression in a dose-dependent manner, which paralleled the decrease in CRH protein levels in culture media. A complete estrogen receptor (ER) antagonist, ICI 182780, not only blocked repression of CRH mRNA levels by E2, but up-regulated CRH mRNA and protein synthesis. An ER alpha-mixed agonist/antagonist and ER beta antagonist, 4-hydroxytamoxifen, also down-regulated CRH gene expression. Using quantitative RT-PCR, we found that placental trophoblasts express predominantly the ER alpha form of the receptor. Transient transfection assays conducted in the choriocarcinoma cell line JEG-3 demonstrated that E2 repressed CRH promoter activity, whereas the antagonist ICI 182780 up-regulated CRH promoter activity when ER alpha was cotransfected. These studies demonstrate that E2 represses placental CRH gene expression through an ER alpha-mediated mechanism. Estrogen may therefore modulate placental CRH production, influencing the rate of rise of maternal plasma CRH concentrations and potentially the length of gestation.