Cadmium (Cd) is a heavy metal that mimics, in vivo, the effect of estrogens in the uterus and mammary gland; In estrogen-responsive breast cancer cell lines, Cd stimulates proliferation and can also activate the estrogen receptor independently of estradiol. We have previously proved that ERa- and ERb-mediated transcription are stimulated by this metal but only Cd-induced ERa-mediated transcription is inhibited by melatonin, whereas ERb-mediated transcription is not reduced by the pineal hormone, in both ERE- and AP1-containing promoters. Telomerase is an enzyme responsible of telomere elongation that is activated in most human cancers. Given that estradiol (E2) induces both expression and activity of the catalytic subunit of telomerase, hTERT, in an ERa-dependent fashion, the main goal of this work was to determine whether melatonin inhibited Cd-induced telomerase expression in MCF7 cells. We found, indeed, that exposure to micromolar concentrations of Cd significantly increased hTERT expression in MCF7 cells. We also observed a significant reduction of hTERT mRNA expression in melatonin-treated cells. In transient transfection experiments, we proved that telomerase expression is stimulated by Cd through both ERa- and ERb but only Cd-induced ERa-mediated transcription is specifically inhibited by the pineal hormone at the hTERT promoter, whereas ERb-mediated transcription is not inhibited by melatonin. These results contribute to shed light on the anti-estrogenic properties of melatonin in breast cancer cell lines.