Capsazepine has been widely used as a selective antagonist of vanilloid type 1 receptors; however, its other in vitro effect on most cell types is unknown. In human PC3 prostate cancer cells, the effect of capsazepine on intracellular Ca 2+ concentrations ([Ca 2+ ] i ) and cytotoxicity was investigated by using fura-2 and tetrazolium, respectively. Capsazepine caused a rapid rise in [Ca 2+ ] i in a concentration-dependent manner with an EC 50 value of 75μM. Capsazepine-induced [Ca 2+ ] i rise was reduced by 60% by removal of extracellular Ca 2+ , suggesting that the capsazepine-induced [Ca 2+ ] i rise was contributed by extracellular Ca 2+ influx and intracellular Ca 2+ . Consistently, the capsazepine (200μM)-induced [Ca 2+ ] i rise was decreased by La 3+ by half. In Ca 2+ -free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca 2+ -ATPase, caused a monophasic [Ca 2+ ] i rise, after which the effect of capsazepine on [Ca 2+ ] i was inhibited by 80%. Conversely, pretreatment with capsazepine partly reduced thapsigargin-induced [Ca 2+ ] i rise. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca 2+ mobilizer)-induced, but not capsazepine-induced, [Ca 2+ ] i rise. These findings suggest that in human PC3 prostate cancer cells, capsazepine increases [Ca 2+ ] i by evoking Ca 2+ influx and releasing Ca 2+ from the endoplasmic reticulum via a phospholiase C-independent manner. Overnight incubation with capsazepine (200μM) killed 37% of cells, which could not be prevented by chelating intracellular Ca 2+ with BAPTA.