The mechanism of action of the anti‐apoptotic oncogene Bcl‐2 is still largely obscure. We have recently shown that the overexpression of Bcl‐2 in HeLa cells reduces the Ca 2+ concentration in the endoplasmic reticulum ([Ca 2+] er) by increasing the passive Ca 2+ leak from the organelle. To investigate whether this Ca 2+ depletion is part of the mechanism of action of Bcl‐2, we mimicked the Bcl‐2 effect on [Ca 2+] er by different pharmacological and molecular approaches. All conditions that lowered [Ca 2+] er protected HeLa cells from ceramide, a Bcl‐2‐sensitive apoptotic stimulus, while treatments that increased [Ca 2+] er had the opposite effect. Surprisingly, ceramide itself caused the release of Ca 2+ from the endoplasmic reticulum and thus [Ca 2+] increased both in the cytosol and in the mitochondrial matrix, paralleled by marked alterations in mitochondria morphology. The reduction of [Ca 2+] er levels, as well as the buffering of cytoplasmic [Ca 2+] changes, prevented mitochondrial damage and protected cells from apoptosis. It is therefore concluded that the Bcl‐2‐dependent reduction of [Ca 2+] er is an important component of the anti‐apoptotic program controlled by this oncogene.