We have previously demonstrated that regulation of estrogen receptor (ER) expression in MCF-7 breast cancer cells is a complex process involving transcriptional and posttranscriptional regulation by estradiol. Treatment of MCF-7 cells with estradiol results in the down-regulation of receptor expression; posttranscriptional suppression of receptor mRNA appears to be the predominant mechanism. To determine whether posttranscriptional regulation of ER gene expression is mediated by an ER-dependent mechanism independent of protein synthesis, we have used the competitive estrogen antagonist, 4-hydroxytamoxifen, and the inhibitor of protein synthesis, cycloheximide, to study regulation of ER mRNA by estradiol. 4-Hydroxytamoxifen had no effect on the steady-state level of receptor mRNA and effectively blocked the suppression of ER mRNA by estradiol. The metabolic inhibitor, cycloheximide, was unable to prevent the estrogen induced decrease in ER mRNA. These data provide evidence that the posttranscriptional suppression of ER expression through estradiol is mediated through the ER independent of protein synthesis. A study of the effects of estradiol on the steady-state levels of nuclear and cytoplasmic receptor mRNA suggest that posttranscriptional suppression is a nuclear event.