The epithelial calcium channel TRPV6 is upregulated in breast carcinoma compared with normal mammary gland tissue. The selective estrogen receptor modulator tamoxifen is widely used in breast cancer therapy. Previously, we showed that tamoxifen inhibits calcium uptake in TRPV6-transfected Xenopus oocytes. In this study, we examined the effect of tamoxifen on TRPV6 function and intracellular calcium homeostasis in MCF-7 breast cancer cells transiently transfected with EYFP-C1-TRPV6. TRPV6 activity was measured with fluorescence microscopy using Fura-2. The basal calcium level was higher in transfected cells compared with nontransfected cells in calcium-containing solution but not in nominally calcium-free buffer. Basal influxes of calcium and barium were also increased. In transfected cells, 10 mumol/L tamoxifen reduced the basal intracellular calcium concentration to the basal calcium level of nontransfected cells. Tamoxifen decreased the transport rates of calcium and barium in transfected cells by 50%. This inhibitory effect was not blocked by the estrogen receptor antagonist, ICI 182,720. Similarly, a tamoxifen-induced inhibitory effect was also observed in MDA-MB-231 estrogen receptor-negative cells. The effect of tamoxifen was completely blocked by activation of protein kinase C. Inhibiting protein kinase C with calphostin C decreased TRPV6 activity but did not alter the effect of tamoxifen. These findings illustrate how tamoxifen might be effective in estrogen receptor-negative breast carcinomas and suggest that the therapeutic effect of tamoxifen and protein kinase C inhibitors used in breast cancer therapy might involve TRPV6-mediated calcium entry. This study highlights a possible role of TRPV6 as therapeutic target in breast cancer therapy.