The chronological changes in intracellular Ca(2+)concentrations ([Ca(2+)](i)) were analysed during heat-induced apoptosis in human lung cancer cell lines LK-2 (squamous cell carcinoma) and LU65A (large cell carcinoma). In LK-2 cells, increased [Ca(2+)](i) levels were maintained at levels between 250-350 nm 9 h after heat-shock. Treatment with BAPTA, an intracellular Ca(2+) chelator, prior to heat-shock, decreased the frequency of heat-induced apoptosis in LK-2, while thapsigargin, a selective endoplasmic reticulum Ca(2+)-ATPase inhibitor, did not change the number of apoptotic cells, regardless of the presence or absence of Ca(2+)-supplemented medium. In LU65A cells, treatment with BAPTA or thapsigargin did not alter the apoptotic rates. Western blotting demonstrated that, although expression of Bax and Bcl-2 were not changed by heat-shock, p53 expression was elevated in LK-2, but not LU65A cells. Immunohistochemistry showed that p53 was localized predominantly in the cytoplasms of LK-2 cells, suggesting that p53 protein is not functional in LK-2. Heat-shock also elevated activities of caspase-3, -8 and -9 in both cell lines. It is concluded that a temporal increase in [Ca(2+)](i) is the important initiating factor in hyperthermia-induced apoptosis in LK-2 cells and that, in these two lung cancer cell lines, apoptosis may occur through 'cross-talk' between p53-independent mitochondrial and death receptor pathways.