Experimentally, a portion of the detectable cellular estrogen receptor (ER) is seen to disappear in human breast cancer cells submitted to estradiol treatment. In this study, we have applied several detection methods to analyze the loss (processing) then the replenishment of ER in the MCF-7 cell line. Radioligand exchange assay and enzyme immunoassay revealed an accumulation of ER in the nuclei with a concomitant depletion in cytosol shortly after the addition of estradiol in cell culture. Then, a time-dependent decrease of ER level in the nuclear compartment without rescue in the cytosol was observed. When an immunocytochemical assay was performed on whole cells treated with estradiol, a similar decrease of ER number was shown, indicating that a decrease in the extractability of estradiol-filled ER was not involved in the processing. Analysis of ER mRNA also indicated that the estrogen treatment induces a time-dependent decrease of its expression. Measurement of [35S]methionine-labeled ER following the arrest of the hormone treatment suggested that ER replenishment was due to newly synthesized receptors. Sucrose gradient experiments confirmed the generation of small molecular forms of ER, following its binding with estradiol. All these data are indicative of estrogen-receptor complex degradation. We also confirm that estrogen regulates ER level through the decrease of its mRNA expression.