Estrogen Receptor (ER) RNA has been studied in ER positive and ER negative cells, grown in alpha medium and fetal calf serum (FCS), with and without estrogen addition. ER mRNA was detected only in ER positive cells. When cells were grown without estrogen addition, a progressive and transient increase of ER RNA levels was observed. When cells were cultured with estradiol, the ER RNA increase took place early and ER RNA levels remained 2 times higher than in untreated cells. Therefore, a down-regulation of ER RNA is not apparent in neoplastic breast cells. We hypothesize that the increase of ER expression is caused by stabilization of ER transcripts, due to the estrogen induced ER cytoplasmic depletion.