We have measured the expression of T-type Ca2+ channel mRNA in breast cancer cell lines (MCF-7 (ERalpha+) using Western blot and quantitative real-time PCR (Q-RT-PCR). These results revealed that the MCF-7 cells express both alpha1G and alpha1H isoforms of T-type Ca2+ channels. In order to further clarify the role of T-type Ca2+ channels in proliferation, we tested the effects of a selective T-type Ca2+ channel inhibitor NNC-55-0396 on cellular proliferation. MCF-7 (ERalpha+) cellular proliferation was inhibited by the compound. In contrast, NNC-55-0396 at same concentration had no effect on the proliferation of MCF-10A cells, a non-cancer breast epithelial cell line. We also found that message expression of the T-type Ca2+ channels were only expressed in rapidly growing non-confluent cells but not in the cytostatic confluent cells. Knocking down the expression of T-type Ca2+ channels with siRNA targeting both alpha1G and alpha1H resulted in growth inhibition as much as 45%+/-5.0 in MCF-7 cells as compared to controls. In conclusion, our results suggest that T-type Ca2+ channel antagonism/silencing may reduce cellular proliferation in mitogenic breast cells.