Enhanced c-myc oncogene expression associated with peptide mitogen-stimulated cell growth is primarily a result of a post-transcriptional event involving increased mRNA stability. We have recently shown that estradiol stimulates c-myc expression in estrogen receptor-positive human breast cancer cells. In this report, we show that in estrogen-responsive MCF-7 cells, estradiol stimulated the c-myc gene exclusively at the transcriptional level, increasing c-myc mRNA transcription more than 10-fold within 20 min, while having no effect on the c-myc mRNA half-life of 18 min. In addition, pretreatment of the cells with cycloheximide did not prevent induction of the c-myc oncogene, indicating a primary effect of estrogen. Furthermore, the elevated level of c-myc mRNA in estrogen-independent MDA-MB-231 cells was due primarily to a more stable c-myc mRNA with a half-life of 49 min, in the absence of enhanced transcription. These results indicate that different mechanisms of regulation of c-myc oncogene expression exist in hormone-dependent and -independent human breast cancer cells.