The antiestrogen tamoxifen is commonly used to treat breast cancer, but it also has therapeutic activity in several other types of cancer. Many of these tumors, including malignant gliomas, are estrogen receptor negative. Nonetheless, high concentrations of tamoxifen can directly reduce cell proliferation in some of these tumors and induce apoptosis. In this study, the role of tamoxifen in calcium signaling and calcium-induced cell death was studied in both malignant glioma cell lines and MCF-7 breast cancer cells. Tamoxifen potently increased the spatial expansion of calcium waves by 30-150% while significantly enhancing and prolonging agonist-induced calcium elevations. Furthermore, tamoxifen pretreatment accelerated calcium ionophore-induced death by more than 20 min, suggesting that tamoxifen lowered cellular resistance to calcium loads. In contrast to its potentiating of calcium signaling in tumors, tamoxifen had no significant effect on calcium signaling in cultures of primary astrocytes from either human or rat brain. This study demonstrates the existence of calcium signaling in breast cancer and glioma cells and identifies tamoxifen as a potential modulator of tumor-associated calcium signaling.