Apoptosis in response to stress signals activates effector caspases known to be regulated by the release of cytochrome c (Cyt c) from mitochondria and the subsequent ATP-dependent activation of the death regulator apoptotic protease-activating factor 1 (Apaf-1). Experiments were carried out to determine whether the release of Cyt c is evoked by NO. in RAW 264.7 macrophages and to position signaling components relative to mitochondria. S-nitrosoglutathione and spermine-NO caused a fast p53 accumulation, followed by Bcl-xL downregulation, Cyt c release, and caspase activation. These alterations were absent in p53 antisense expressing macrophages (R delta p53asn-11). In Bcl-2 overexpressing cells (Rbcl2-14) Cyt c relocation and caspase activation were abrogated although p53 accumulation remained intact. The use of caspase inhibitors revealed Cyt c release and decreased Bcl-xL expression to be caspase independent. ATP-depleted cells showed a shift from apoptosis towards necrosis and no p53 accumulation or caspase activation upon NO. addition. Conclusively, NO.-mediated apoptosis in macrophages is entirely controlled by the mitochondrial pathway with the implication that Cyt c relocation demands p53 accumulation. Moreover, pulse-chase-experiments in combination with the ATP-depletion protocol identified p53 accumulation and stabilization as an energy requiring process. This allowed to dissect two ATP-dependent steps, one is in association with Apaf-1 formation, while the other resides in p53 accumulation.