Inositol 1,4,5-trisphosphate receptor-deficient (IP3RKO) B-lymphocytes were used to investigate the functional relevance of type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) and its cleavage by caspase-3 in apoptosis. We showed that inositol 1,4,5-trisphosphate receptor-deficient cells were largely resistant to apoptosis induced by both staurosporine (STS) and B-cell receptor (BCR) stimulation. Expression of either the wild-type IP3R1 or an N-terminal deletion mutant (Delta1-225) that lacks inositol 1,4,5-trisphosphate-induced Ca2+ release activity restored sensitivity to apoptosis and the consequent rise in free cytosolic Ca2+ concentration ([Ca2+]i). Expression of caspase-3-non-cleavable mutant receptor, however, dramatically slowed down the rate of apoptosis and prevented both Ca2+ overload and secondary necrosis. Conversely, expression of the "channel-only" domain of IP3R1, a fragment of the receptor generated by caspase-3 cleavage, strongly increased the propensity of the cells to undergo apoptosis. In agreement with these observations, caspase inhibitors impeded apoptosis and the associated rise in [Ca2+]i. Both the staurosporine- and B-cell receptor-induced apoptosis and increase in [Ca2+]i could be induced in nominally Ca2+-free and serum-free culture media, suggesting that the apoptosis-related rise in [Ca2+]i was primarily because of the release from internal stores rather than of influx through the plasma membrane. Altogether, our results suggest that IP3R1 plays a pivotal role in apoptosis and that the increase in [Ca2+]i during apoptosis is mainly the consequence of IP3R1 cleavage by caspase-3. These observations also indicate that expression of a functional IP3R1 per se is not enough to generate the significant levels of cytosolic Ca2+ needed for the rapid execution of apoptosis, but a prior activation of caspase-3 and the resulting truncation of the IP3R1 are required.