Free intracellular calcium ([Ca²⁺]_i), in addition to being an important second messenger, is a key regulator of many cellular processes including cell membrane potential, proliferation, and apoptosis. In many cases, the mobilization of ([Ca²⁺]_i) is controlled by intracellular store activation and calcium influx. We have investigated the effect of several ion channel modulators, which have been used to treat a range of human diseases, on ([Ca²⁺]_i) release, by ratiometric calcium imaging. We show that six such modulators [amiodarone (Ami), dofetilide, furosemide (Fur), minoxidil (Min), loxapine (Lox), and Nicorandil] initiate release of ([Ca²⁺]_i) in prostate and breast cancer cell lines, PC3 and MCF7, respectively. Whole-cell currents in PC3 cells were inhibited by the compounds tested in patch-clamp experiments in a concentration-dependent manner. In all cases ([Ca²⁺]_i) was increased by modulator concentrations comparable to those used clinically. The increase in ([Ca²⁺]_i) in response to Ami, Fur, Lox, and Min was reduced significantly (P < 0.01) when the external calcium was reduced to nM concentration by chelation with EGTA. The data suggest that many ion channel regulators mobilize ([Ca²⁺]_i). We suggest a mechanism whereby calcium-induced calcium release is implicated; such a mechanism may be important for understanding the action of these compounds.