Selective serotonin reuptake inhibitors (SSRIs), a group of antidepressants, are generally used for treatment of various mood and anxiety disorders. There has been much research showing the anti-tumor and cytotoxic activities of some antidepressants; but the detailed mechanisms were unclear. In cultured human osteosarcoma cells (MG63), paroxetine reduced cell viability in a concentration- and time-dependent manner. Paroxetine caused apoptosis as assessed by propidium iodide-stained cells and increased caspase-3 activation. Although immunoblotting data revealed that paroxetine could activate the phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), only SB203580 (a p38 MAPK inhibitor) partially prevented cells from apoptosis. Paroxetine also induced [Ca2+]i increases which involved the mobilization of intracellular Ca2+ stored in the endoplasmic reticulum and Ca2+ influx from extracellular medium. However, pretreatment with BAPTA/AM, a Ca2+ chelator, to prevent paroxetine-induced [Ca2+]i increases did not protect cells from death. The results suggest that in MG63 cells, paroxetine caused Ca2+-independent apoptosis via inducing p38 MAPK-associated caspase-3 activation.