Fluoxetine is a serotonin-specific reuptake inhibitor that has been used as an antidepressant. This study examined the effect of fluoxetine on cytosolic free Ca²⁺ concentrations ([Ca2⁺]i) and viability in OC2 human oral cancer cells. The Ca²⁺-sensitive fluorescent dye fura-2 was used to measure [Ca²⁺]i, and the water soluble tetrazolium (WST-1) regent was used to measure viability. Fluoxetine induced [Ca²⁺]i rises concentration-dependently. The response was reduced by half by removing extracellular Ca²⁺. Fluoxetine-induced Ca²⁺ entry was enhanced by activation of protein kinase C (PKC) with phorbol 12-myristate 13 acetate (PMA) but was inhibited by inhibition of the enzyme with GF109203X. In Ca²⁺-free medium, treatment with the endoplasmic reticulum Ca²⁺ pump inhibitor 2,5-di-tert-butylhydroquinone (BHQ) or thapsigargin abolished fluoxetine-evoked [Ca²⁺]i rise. Conversely, treatment with fluoxetine inhibited BHQ/thapsigargin-evoked [Ca²⁺]i rise. Inhibition of phospholipase C (PLC) with U73122 abolished fluoxetine-induced [Ca²⁺]i rise. At 20-80 μM, fluoxetine decreased cell viability concentration-dependently, which was not altered by chelating cytosolic Ca²⁺ with 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM). At 20-60 μM, fluoxetine induced apoptosis as detected by annexin V/propidium iodide (PI) staining. Together, in OC2 cells, fluoxetine induced [Ca²⁺]i rises by evoking PLC-dependent Ca²⁺ release from the endoplasmic reticulum and Ca²⁺ entry via PKC-regulated mechanisms. Fluoxetine also caused Ca²⁺-independent apoptosis.