The signaling events leading to apoptosis can be divided into two major pathways, involving either mitochondria (intrinsic) or death receptors (extrinsic). In a recent study, we have shown the involvement of the mitochondria-dependent apoptotic pathway in heat-induced male germ cell apoptosis in the rat. In additional studies, using the gld (generalized lymphoproliferation disease) and lpr^cg (lymphoproliferation complementing gld) mice, which harbor loss-of-function mutations in Fas L and Fas, respectively, we have shown that heat-induced germ cell apoptosis is not blocked, thus providing evidence that the Fas signaling system is not required for heat-induced germ cell apoptosis in the testis. In the present study, we have found that the initiation of apoptosis in wild-type mice was preceded by a redistribution of Bax from a cytoplasmic to paranuclear localization in heat-susceptible germ cells. The relocation of Bax is accompanied by sequestration of ultracondensed mitochondria into paranuclear areas of apoptotic germ cells, cytosolic translocation of mitochondrial cytochrome c and DIABLO, and is associated with activation of the initiator caspase 9 and the executioner caspase 3. Similar events were also noted in both gld and lpr^cg mice. Taken together, these results indicate that the mitochondria-dependent pathway is the key apoptotic pathway for heat-induced male germ cell death in mice.