The roles of oxygen and reactive oxygen intermediates in apoptosis are unclear at present. Although oxygen and reactive oxygen intermediates are not required for the execution of apoptosis, oxygen may be involved in at least some forms of apoptosis. In this study we show that dexamethasone (Dex)-induced apoptosis of immature mouse thymocytes is completely inhibited by hypoxic culture. In contrast, anti-CD95 thymocyte apoptosis is unaffected by hypoxia, indicating the existence of two forms of thymocyte apoptosis: an oxygen-dependent pathway (Dex induced) and an oxygen-independent pathway (anti-CD95 induced). Furthermore, hypoxia inhibited mitochondrial permeability transition (PT) in Dex-treated, but not in anti-CD95-treated, thymocytes, suggesting that the oxygen-sensitive step is upstream of mitochondria. Both Dex-and anti-CD95-induced PT and apoptosis were dependent on activation of IL-converting enzyme-like protease, as PT and apoptosis were inhibited by preincubation with Cbz-Val-Ala-Asp-fluoromethyl ketone, an irreversible inhibitor of IL-converting enzyme-like proteases. In addition, hypoxia inhibited the activation by Dex of caspase-3 (CPP32)-like proteases. Our data show that the private signaling pathways of Dex (oxygen dependent) and anti-CD95 (oxygen independent) both converge upstream of mitochondrial changes. The oxygen-dependent step in Dex-induced apoptosis lies upstream of caspase-3-like protease activation. Our observations support a model of apoptosis signaling in which independent pathways (oxygen dependent and oxygen independent) particular to each stimuli converge at a central point in the apoptotic cascade.