We recently reported that nonsteroidal anti-inflammatory drug (NSAID)-induced gastric lesions involve NSAID-induced apoptosis of gastric mucosal cells, which in turn involves the endoplasmic reticulum stress response, in particular the up-regulation of CCAAT/enhancer-binding protein homologous transcription factor (CHOP). In this study, we have examined the molecular mechanism governing this NSAID-induced apoptosis in primary cultures of gastric mucosal cells. Various NSAIDs showed membrane permeabilization activity that correlated with their apoptosis-inducing activity. Various NSAIDs, particularly celecoxib, also increased intracellular Ca2+ levels. This increase was accompanied by K+ efflux from cells and was virtually absent when extracellular Ca2+ had been depleted. These data indicate that the increase in intracellular Ca2+ levels that is observed in the presence of NSAIDs is due to the stimulation of Ca2+ influx across the cytoplasmic membrane, which results from their membrane permeabilization activity. An intracellular Ca2+ chelator partially inhibited celecoxib-induced release of cytochrome c from mitochondria, reduced the magnitude of the celecoxib-induced decrease in mitochondrial membrane potential and inhibited celecoxib-induced apoptotic cell death. It is therefore likely that an increase in intracellular Ca2+ levels is involved in celecoxib-induced mitochondrial dysfunction and the resulting apoptosis. An inhibitor of calpain, a Ca2+-dependent cysteine protease, partially suppressed mitochondrial dysfunction and apoptosis in the presence of celecoxib. Celecoxib-dependent CHOP-induction was partially inhibited by the intracellular Ca2+ chelator but not by the calpain inhibitor. These results suggest that Ca2+-stimulated calpain activity and CHOP expression play important roles in celecoxib-induced apoptosis in gastric mucosal cells.